C++11 : Lambda functions return value

[Alexandre]

Hey there. I installed the newest GCC version yesterday to try out the new C++ standard. I have added the command line argument so that it knows to use C++11 and compiled a sample code from CProgramming's webmaster successfully. I wanted to play around a little bit to learn how to use the new tools but it seems I can't make anything compile:

#include <iostream>

using namespace std;

void blah(int a) {
    std::cout << a;
}

int main()
{
    auto a = [] { return 1; };
    
    blah(a);
    blah([] { return 1; });
    blah([] -> int { return 1; });
}

"/usr/bin/make" -f nbproject/Makefile-Debug.mk QMAKE= SUBPROJECTS= .build-conf
make[1]: entrant dans le répertoire « /cygdrive/c/Users/win7/Documents/NetBeansProjects/CppApplication_1 »
"/usr/bin/make"  -f nbproject/Makefile-Debug.mk dist/Debug/Cygwin_4.x-Windows/cppapplication_1.exe
make[2]: entrant dans le répertoire « /cygdrive/c/Users/win7/Documents/NetBeansProjects/CppApplication_1 »
mkdir -p build/Debug/Cygwin_4.x-Windows
rm -f build/Debug/Cygwin_4.x-Windows/main.o.d
g++-4.exe -std=c++0x -Wall -Wextra   -c -g -MMD -MP -MF build/Debug/Cygwin_4.x-Windows/main.o.d -o build/Debug/Cygwin_4.x-Windows/main.o main.cpp
main.cpp: In function ‘int main()’:
main.cpp:13:11: erreur: invalid conversion from ‘int (*)()’ to ‘int’
main.cpp:13:11: erreur:   initializing argument 1 of ‘void blah(int)’
main.cpp:14:26: erreur: invalid conversion from ‘int (*)()’ to ‘int’
main.cpp:14:26: erreur:   initializing argument 1 of ‘void blah(int)’
main.cpp:15:36: erreur: invalid conversion from ‘int (*)()’ to ‘int’
main.cpp:15:36: erreur:   initializing argument 1 of ‘void blah(int)’
make[2]: quittant le répertoire « /cygdrive/c/Users/win7/Documents/NetBeansProjects/CppApplication_1 »
make[1]: quittant le répertoire « /cygdrive/c/Users/win7/Documents/NetBeansProjects/CppApplication_1 »
make[2]: *** [build/Debug/Cygwin_4.x-Windows/main.o] Erreur 1
make[1]: *** [.build-conf] Erreur 2
make: *** [.build-impl] Erreur 2

BUILD FAILED (exit value 2, total time: 2s)

Obviously this is all the same syntax error, but I do not understand what my error is. From what I have seen online it seems that this is the way to do this :\

[Shakti]

auto a = [] { return 1; };

Needs to be

auto a = []() { return 1; };

blah(a);

Since blah takes an int you need to use a as a function:

blah(a());

Now to have a function take a lambda function as an argument you need to use std::function:

#include <iostream>
#include <functional>

int foo(std::function<int()> func)
{
    return func();
}

int main()
{
    auto f = []() { return 1; };
    std::cout << foo(f) << std::endl;
}

Note the template argument to std::function, it specifies the returntype and the argument list types. If the lambda function should take an int as an argument we modify it the std::function declaration as such:

#include <iostream>
#include <functional>

int foo(std::function<int(int)> func)
{
    return func(1);
}

int main()
{
    auto f = [](int a) { return 2*a; };
    std::cout << foo(f) << std::endl;
}

[Elysia]

You can use templates, too, which might be preferred since std::function has some overhead.

[Alexandre]

It seems that I hadn't been reading the tutorial through. I modified my code to the following and it works fine:

#include <iostream>
#include <functional>

void blah(std::function<int(void)> func) {
    std::cout << func();
}

int main()
{
    auto a = []()->int { return 1; };
    blah(a);
    blah([] { return 1; });
    blah([] () -> int { return 1; });
}

[whiteflags]

You can use templates, too, which might be preferred since std::function has some overhead.

And you might prefer std::function, too, because it allows you to declare the function in one place and define it somewhere else easily. Unlike template versions which are implemented in h files.

[Alexandre]

This example was strictly designed for me to try the new Lambda functions feature so templates are, of course, not relevant at all unless there is some point I am missing.

[whiteflags]

One way to give another function a lambda function is through template parameters. I believe the tutorial covers this.