How can I use C++14 features when building qmake projects?

Question

I'm currently using C++11 features in my Qt applications. However, I'd like to use some of the new C++14 features in my applications.

To enable C++11 in a Qt application, one only needs to add one line in the qmake project file, namely:

CONFIG += c++11

or this for earlier versions:

QMAKE_CXXFLAGS += -std=c++1y

I already tried to do the same with C++14, but it didn't work. I changed the above mentioned line of the qmake project like this:

CONFIG += c++14

or this for earlier versions:

QMAKE_CXXFLAGS += -std=c++1y

After that, lots of compilation errors, that did not exist before, appear when trying to build the project. The project compiles fine, however, if I try to use any C++14 features, I get a compilation error. This is an example:

template<typename T>
constexpr T pi = T(3.1415926535897932385);

This is the corresponding error:

main.cpp:7: error: template declaration of 'constexpr const T pi'
constexpr T pi = T(3.1415926535897932385);  
          ^

How to enable C++14 features when using a qmake project in QtCreator?

I am using Qt 5.3.2, Qt Creator 3.2.1, and MinGW 4.8.2 32 bit.

Solution 1

Qt Creator is just an IDE.

You can think of IDEs as "smarter text editors" that aid the developer with debugging, building, code completion, file management and so on.

IDEs are irrelevant during compilation.

What matters is your compiler. And it is independent from your IDE.

g++ 4.8.x does not support many C++14 features: check out this page to learn what C++14 features are supported.

Solution 2

This is now supported properly from Qt 5.4. You just type this into your qmake project file:

CONFIG += c++14

The necessary code change went in the middle of 2014 by Thiago:

Add support for CONFIG += c++14

I have just created the necessary documentation change:

Mention the c++14 CONFIG option in the qmake variable reference

Please note that variable templates are only supported from gcc 5.0, but then your code works just fine. This can be used for testing C++14 with older gcc:

main.cpp

#include <iostream>

int main()
{
    // Binary literals: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3472.pdf
    // Single-quotation-mark as a digit separator: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3781.pdf
    std::cout << 0b0001'0000'0001;
    return 0;
}

main.pro

TEMPLATE = app
TARGET = main
CONFIG -= qt
CONFIG += c++14
SOURCES += main.cpp

Build and Run

qmake && make && ./main

Output

257

Solution 3

For some weird reason this is what Qt does: If the compiler is gcc or mingw, CONFIG+=C++14 is transformed to a compiler flag -std=c++14 (that what you expect)

If it's another compiler (like clang), CONFIG=C++14 is stupidly transformed to -std=c++11 (sic!), so you will get errors about unsupported features, even if your clang version correctly supports C++14.

To fix it, specify the flag explicitly:

QMAKE_CXXFLAGS += -std=c++14

This way, you're sure that your compiler (g++, mingw or clang) will receive the correct flags.

Solution 4

To use C++14 with qmake with versions before Qt 5.4 (it doesn't make any difference wether you use it with Qt Creator or with some other IDE or from command line) and gcc, add this to your .pro file:

QMAKE_CXXFLAGS += -std=c++1y

qmake got support for CONFIG += c++14 with Qt 5.4, so you can use that for projects where you are comfortable with requiring at least that version of Qt. Be sure to either use the explicit compiler flags, or use the CONFIG option, but not both at the same time, to avoid conflicting switches.

---

Unfortunately gcc 4.8.2 supports very few C++14 features (see here), but you can test that with this code snippet:

#include <iostream>

auto f() { return 42; }

int main()
{
    std::cout << f() << std::endl;
}

That will compile fine with QMAKE_CXXFLAGS += -std=c++1y, but give warning with CONFIG += c++11 or QMAKE_CXXFLAGS += -std=c++1x.

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