This way of initializing a struct sockaddr_in uses modern C and avoids all the raw of days that should long be forgotten, at least for most applications. I use this approach successfully e.g. for ESP32 microcontrollers.
struct sockaddr_in server_addr = {
.sin_family = AF_INET,
.sin_port = htons(46118),
.sin_addr = {.s_addr = htonl(INADDR_ANY)}
};
This example code takes care of mounting the filesystem, or creating a filesystem if none is present.
#pragma once #include <stddef.h> // InitFilesystem() sets this to true if the filesystem is available. extern volatile bool filesystemOK; void InitFilesystem();
#include "FS.h"
#include "LittleFS.h"
volatile bool filesystemOK = false;
void InitFilesystem() {
// Initialize LittleFS
if (!LittleFS.begin(false /* false: Do not format if mount failed */)) {
Serial.println("Failed to mount LittleFS");
if (!LittleFS.begin(true /* true: format */)) {
Serial.println("Failed to format LittleFS");
} else {
Serial.println("LittleFS formatted successfully");
filesystemOK = true;
}
} else { // Initial mount success
filesystemOK = true;
}
}
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(int argc, char** argv) {
std::string s = "My string";
// This reverses s in-place.
std::reverse(std::begin(s), std::end(s));
// Prints "gnirts yM"
cout << s << endl;
}
This minimal example shows how to replace strings using the re2 GlobalReplace() function:
#include <re2/re2.h>
#include <iostream>
#include <string>
using namespace re2;
using namespace std;
// Pre-compiled regex
RE2 myRegex("\\$[^\\$]+\\$");
int main(int argc, char** argv) {
string myString = "This string $abc123$ contains two formulas $123 != 456$.";
// NOTE: This will modify myString!
RE2::GlobalReplace(&myString, myRegex, "FORMULA");
// Prints "This string FORMULA contains two formulas FORMULA."
cout << myString << endl;
}
#include <re2/re2.h> using namespace re2;
This example shows how to get a query parameter in cpp-httplib. See the cpp-httplib hello example for the full source code:
svr.Get("/hi", [](const httplib::Request& req, httplib::Response& res) {
std::string id = req.get_param_value("id");
res.set_content("id=" + id, "text/plain");
});Access it at http://localhost:8080/hi?id=myid
and it will show you
id=myid
id will be empty (== "") if no such parameter is given.
When trying to build your C++ application using Paho MQTT, you see a bunch of undefined reference error messages:
MQTTAsync.c:(.text+0x3482): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34a1): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34c0): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34df): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34fe): undefined reference to `OpenSSL_version' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(MQTTAsyncUtils.c.o): in function `MQTTAsync_closeOnly': MQTTAsyncUtils.c:(.text+0x9b6): undefined reference to `SSL_SESSION_free' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(MQTTAsyncUtils.c.o): in function `MQTTAsync_receiveThread': MQTTAsyncUtils.c:(.text+0x463b): undefined reference to `SSL_set_session' /usr/bin/ld: MQTTAsyncUtils.c:(.text+0x5255): undefined reference to `SSL_get1_session' /usr/bin/ld: MQTTAsyncUtils.c:(.text+0x55bb): undefined reference to `SSL_get1_session' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `call_ssl_psk_cb': SSLSocket.c:(.text+0x11b): undefined reference to `SSL_get_SSL_CTX' /usr/bin/ld: SSLSocket.c:(.text+0x129): undefined reference to `SSL_CTX_get_ex_data' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_error': SSLSocket.c:(.text+0x1fa): undefined reference to `SSL_get_error' /usr/bin/ld: SSLSocket.c:(.text+0x22d): undefined reference to `ERR_print_errors_cb' /usr/bin/ld: SSLSocket.c:(.text+0x2f1): undefined reference to `ERR_get_error' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSL_CTX_info_callback': SSLSocket.c:(.text+0x376): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x381): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x3bc): undefined reference to `SSL_get_verify_result' /usr/bin/ld: SSLSocket.c:(.text+0x411): undefined reference to `SSL_state_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x461): undefined reference to `SSL_get_current_cipher' /usr/bin/ld: SSLSocket.c:(.text+0x469): undefined reference to `SSL_CIPHER_get_name' /usr/bin/ld: SSLSocket.c:(.text+0x474): undefined reference to `SSL_state_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x4d1): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x4dc): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x4e7): undefined reference to `SSL_state_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x503): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x50e): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x541): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x54c): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_initialize': SSLSocket.c:(.text+0x8ee): undefined reference to `OPENSSL_init_ssl' /usr/bin/ld: SSLSocket.c:(.text+0x90a): undefined reference to `OPENSSL_init_crypto' /usr/bin/ld: SSLSocket.c:(.text+0x916): undefined reference to `OPENSSL_init_ssl' /usr/bin/ld: SSLSocket.c:(.text+0x922): undefined reference to `OPENSSL_init_crypto' /usr/bin/ld: SSLSocket.c:(.text+0xa11): undefined reference to `CRYPTO_get_ex_new_index' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_createContext': SSLSocket.c:(.text+0xbee): undefined reference to `SSL_CTX_use_certificate_chain_file' /usr/bin/ld: SSLSocket.c:(.text+0xc48): undefined reference to `SSL_CTX_set_default_passwd_cb' /usr/bin/ld: SSLSocket.c:(.text+0xc55): undefined reference to `SSL_CTX_set_default_passwd_cb_userdata' /usr/bin/ld: SSLSocket.c:(.text+0xc67): undefined reference to `SSL_CTX_use_PrivateKey_file' /usr/bin/ld: SSLSocket.c:(.text+0xc9a): undefined reference to `SSL_CTX_load_verify_locations' /usr/bin/ld: SSLSocket.c:(.text+0xce5): undefined reference to `SSL_CTX_set_default_verify_paths' /usr/bin/ld: SSLSocket.c:(.text+0xd2e): undefined reference to `SSL_CTX_set_cipher_list' /usr/bin/ld: SSLSocket.c:(.text+0xd56): undefined reference to `SSL_CTX_set_ex_data' /usr/bin/ld: SSLSocket.c:(.text+0xd66): undefined reference to `SSL_CTX_set_psk_client_callback' /usr/bin/ld: SSLSocket.c:(.text+0xd8f): undefined reference to `SSL_CTX_ctrl' /usr/bin/ld: SSLSocket.c:(.text+0xdc5): undefined reference to `SSL_CTX_free' /usr/bin/ld: SSLSocket.c:(.text+0xe51): undefined reference to `SSL_CTX_set_alpn_protos' /usr/bin/ld: SSLSocket.c:(.text+0xeb9): undefined reference to `TLS_client_method' /usr/bin/ld: SSLSocket.c:(.text+0xec1): undefined reference to `SSL_CTX_new' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_setSocketForSSL': SSLSocket.c:(.text+0x1046): undefined reference to `SSL_CTX_set_info_callback' /usr/bin/ld: SSLSocket.c:(.text+0x1056): undefined reference to `SSL_CTX_set_msg_callback' /usr/bin/ld: SSLSocket.c:(.text+0x1075): undefined reference to `SSL_new' /usr/bin/ld: SSLSocket.c:(.text+0x10ab): undefined reference to `SSL_get_cipher_list' /usr/bin/ld: SSLSocket.c:(.text+0x10be): undefined reference to `SSL_set_fd' /usr/bin/ld: SSLSocket.c:(.text+0x113b): undefined reference to `SSL_ctrl' /usr/bin/ld: SSLSocket.c:(.text+0x121c): undefined reference to `SSL_CTX_set_verify' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_connect': SSLSocket.c:(.text+0x12b7): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x12bf): undefined reference to `SSL_connect' /usr/bin/ld: SSLSocket.c:(.text+0x135d): undefined reference to `SSL_get_peer_certificate' /usr/bin/ld: SSLSocket.c:(.text+0x138c): undefined reference to `X509_check_host' /usr/bin/ld: SSLSocket.c:(.text+0x13d4): undefined reference to `CRYPTO_free' /usr/bin/ld: SSLSocket.c:(.text+0x13f6): undefined reference to `X509_free' /usr/bin/ld: SSLSocket.c:(.text+0x144d): undefined reference to `X509_check_ip_asc' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_getch': SSLSocket.c:(.text+0x1569): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1579): undefined reference to `SSL_read' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_destroyContext': SSLSocket.c:(.text+0x1630): undefined reference to `SSL_CTX_free' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_close': SSLSocket.c:(.text+0x16b8): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x16c1): undefined reference to `SSL_shutdown' /usr/bin/ld: SSLSocket.c:(.text+0x16ce): undefined reference to `SSL_free' /usr/bin/ld: SSLSocket.c:(.text+0x16fd): undefined reference to `SSL_CTX_free' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_putdatas': SSLSocket.c:(.text+0x191f): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1930): undefined reference to `SSL_write' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_getdata': SSLSocket.c:(.text+0x1ca5): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1cb8): undefined reference to `SSL_read' /usr/bin/ld: SSLSocket.c:(.text+0x1d7f): undefined reference to `SSL_pending' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_continueWrite': SSLSocket.c:(.text+0x1e2f): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1e3f): undefined reference to `SSL_write' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(Base64.c.o): in function `Base64_encodeDecode.part.0': Base64.c:(.text+0x1f): undefined reference to `BIO_f_base64' /usr/bin/ld: Base64.c:(.text+0x27): undefined reference to `BIO_new' /usr/bin/ld: Base64.c:(.text+0x2f): undefined reference to `BIO_s_mem' /usr/bin/ld: Base64.c:(.text+0x37): undefined reference to `BIO_new' /usr/bin/ld: Base64.c:(.text+0x45): undefined reference to `BIO_push' /usr/bin/ld: Base64.c:(.text+0x55): undefined reference to `BIO_set_flags' /usr/bin/ld: Base64.c:(.text+0x78): undefined reference to `BIO_write' /usr/bin/ld: Base64.c:(.text+0x8e): undefined reference to `BIO_ctrl' /usr/bin/ld: Base64.c:(.text+0x9e): undefined reference to `BIO_free_all' /usr/bin/ld: Base64.c:(.text+0xc1): undefined reference to `BIO_read' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `WebSocket_buildFrame.constprop.0': WebSocket.c:(.text+0x441): undefined reference to `RAND_bytes' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `uuid_generate': WebSocket.c:(.text+0x721): undefined reference to `RAND_bytes' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `WebSocket_connect': WebSocket.c:(.text+0x929): undefined reference to `RAND_bytes' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `WebSocket_upgrade': WebSocket.c:(.text+0x1dd9): undefined reference to `SHA1_Init' /usr/bin/ld: WebSocket.c:(.text+0x1def): undefined reference to `SHA1_Update' /usr/bin/ld: WebSocket.c:(.text+0x1dff): undefined reference to `SHA1_Final' collect2: error: ld returned 1 exit status
In addition to linking the Paho MQTT libraries, you also need to link OpenSSL, i.e. the ssl and crypto libraries. You can link them, for example using
target_link_libraries(myexecutable ssl crypto)
in CMake or by adding these linker flags for GCC:
-lssl -lcrypto
target_link_libraries(myexecutable paho-mqttpp3 paho-mqtt3as ssl crypto)
Important note: This post is for Conan 1.x and is not compatible with the current version conan 2.x! See How to initialize C++ project using conan 2.x for an updated version of this post!
In the directory where you want to create the project, run (obviously, replace mypackageby the name of everywhere in our code!)
conan new mypackage/1.0.0 -s
-s is important here. It tells conan that the source is available in the project directory and it should not be pulled using git etc.
This will initialize a shared library project, which we will now modify to build an executable.
First, open src/CMakeLists.txt and change add_library() to add_executable(), for example
add_library(mypackage mypackage.cpp)
to
add_executable(mypackage mypackage.cpp)
Now we shall modify conanfile.py to properly build & install the executable:
def package(self):
self.copy("*.h", dst="include", src="src")
self.copy("*.lib", dst="lib", keep_path=False)
self.copy("*.dll", dst="bin", keep_path=False)
self.copy("*.dylib*", dst="lib", keep_path=False)
self.copy("*.so", dst="lib", keep_path=False)
self.copy("*.a", dst="lib", keep_path=False)
def package_info(self):
self.cpp_info.libs = ["NoxecoDB"]
with
def package(self):
self.copy("mypackage", src="bin", dst="bin", keep_path=False)
def package_info(self):
self.env_info.PATH = os.path.join(self.package_folder, "bin")
def deploy(self):
self.copy("mypackage", dst="bin", src="bin")
and add
import os.path
to the top of the file
Now let’s add a main() function to src/mypackage.cpp. Replace the file’s content with
#include <iostream>
int main(int argc, char** argv) {
std::cout << "Hello World!" <<std::endl;
}
Now it’s time to install the dependencies & build the project using
conan install . && conan build .
The executable will be in
bin/mypackage
You can run it using
./bin/mypackage
which will print
Hello World!
I also recommend to add this .gitignore:
CMakeCache.txt CMakeFiles/ Makefile bin/ cmake_install.cmake conan.lock conanbuildinfo.cmake conanbuildinfo.txt conaninfo.txt graph_info.json
#include <boost/algorithm/string/split.hpp> // boost::algorithm::split
#include <boost/algorithm/string/classification.hpp> // boost::is_any_of
#include <string>
#include <iostream>
#include <vector>
int main() {
std::string mystr = "foo-bar-x";
// Split by "-"
std::vector<std::string> splitResult;
boost::algorithm::split(splitResult, mystr, boost::is_any_of("-"));
// Print results, one line at a time
// Prints:
// foo
// bar
// x
for(const std::string& s : splitResult) {
std::cout << s << std::endl;
}
}
You do not need to link the boost::algorithm library. It is a header only library, hence you only need to #include its header files.
For examples, see
#include <boost/algorithm/string/predicate.hpp>
#include <string>
#include <iostream>
int main() {
std::string mystr = "foobar";
// Prints "true, false"
std::cout << std::boolalpha
<< boost::algorithm::starts_with(mystr, "foo") << ", "
<< boost::algorithm::starts_with(mystr, "bar") << std::endl;
}Also see: How to use boost string algorithm ends_with: A minimal example
#include <boost/algorithm/string/predicate.hpp>
#include <string>
#include <iostream>
int main() {
std::string mystr = "foobar";
// Prints "false, true"
std::cout << std::boolalpha
<< boost::algorithm::ends_with(mystr, "foo") << ", "
<< boost::algorithm::ends_with(mystr, "bar") << std::endl;
}Also see: How to use boost string algorithm starts_with: A minimal example
When running conanfile.py or other Python scripts, you see an error message like
Traceback (most recent call last):
File "/home/myuser/myproject/conanfile.py", line 1, in <module>
from conans import ConanFile, CMake
ModuleNotFoundError: No module named 'conans'
Install conan using one of the recommended methods such as the recommended install via pip
sudo pip3 install conan
While trying to compile your project, you see an error message like
error: 'DynamicJSONDocument' was not declared in this scope
The Json in DynamicJsonDocument must be spelled camel case: Json, not JSON! The correct spelling is DynamicJsonDocument.
If that does not help, ensure
#include <ArduinoJson.h>
is at the top of the file where the error occurs.
You must always use 64 bit timestamps for milliseconds since epochs! 32 bits in milliseconds will only cover 1.6 months!
This is how you create and use a task queue in FreeRTOS:
Declare the structure of a task (I recommend to use a task type enum class in order to keep the flexibility of using multiple task types:
#include <freertos/queue.h>
enum class I2CTaskType : uint8_t {
MyTaskType = 0
};
struct I2CTask {
I2CTaskType type;
// Parameters
int16_t value;
};
static QueueHandle_t i2cTaskQueue;Call this once, before using it:
// Create task queue i2cTaskQueue = xQueueCreate(8 /* Number of queue slots */, sizeof(I2CTask));
if (xQueueReceive(i2cTaskQueue, (void *)&task, portMAX_DELAY /* Wait infinitely for new tasks */) == pdTRUE) {
if(task.type == I2CTaskType::MyTaskType) {
// TODO process task
Serial.printf("My task type: %d\r\n", task.value);
}
}void AddTask(int16_t val) {
I2CTask task;
task.type = I2CTaskType::MyTaskType;
task.value = val;
xQueueSend(i2cTaskQueue, (void*)&task, 10 / portTICK_PERIOD_MS /* timeout */);
}
In Arduino, you see an error message like
src/main.cpp:7:48: error: 'size_t' has not been declared void MyFunction(size_t size);
Include stddef.h where size_t is declared:
#include <stddef.h>
Add this line to the top of the file where the error occured.
You are trying to call Wire.begin() for I2C using
Wire.begin(Pin_I2C_SDA, Pin_I2C_SCL, 400000);
but you see an error message like
src/MyI2C.cpp: In function 'void MyInitI2C()':
src/NyI2C.cpp:139:47: error: call of overloaded 'begin(const int&, const int&, int)' is ambiguous
Wire.begin(Pin_I2C_SDA,Pin_I2C_SCL, 400000);
^
In file included from include/MyIO.hpp:2:0,
from src/MyI2C.cpp:2:
/home/uli/.platformio/packages/framework-arduinoespressif32/libraries/Wire/src/Wire.h:79:10: note: candidate: bool TwoWire::begin(int, int, uint32_t)
bool begin(int sda=-1, int scl=-1, uint32_t frequency=0); // returns true, if successful init of i2c bus
^
/home/uli/.platformio/packages/framework-arduinoespressif32/libraries/Wire/src/Wire.h:80:10: note: candidate: bool TwoWire::begin(uint8_t, int, int, uint32_t)
bool begin(uint8_t slaveAddr, int sda=-1, int scl=-1, uint32_t frequency=0);This happens with specific versions of the Arduino framework. For me it happened specifically when upgrading to arduino-esp32 version 2.0.1.
You need to explicitly cast the third argument (400000) to uint32_t in order to tell the compiler which of the two functions you want to call:
Wire.begin(Pin_I2C_SDA, Pin_I2C_SCL, 400000);
Simply use && like this:
std::enable_if<CONDITION1::value && CONDITION2::value, T>
Full example:
template<class T, class I>
constexpr typename std::enable_if<std::is_enum<T>::value && std::is_integral<I>::value, I>::type operator|(T lhs, I rhs)
{
return static_cast<typename std::underlying_type<T>::type>(lhs) | rhs;
}