c++ - 为什么我得到 "error: ‘apply’ is not member of ‘std’”？

Question

我尝试从 cppreference.com 编译这段代码:

#include <iostream>
#include <tuple>
#include <utility>

int add(int first, int second) { return first + second; }

template<typename T>
T add_generic(T first, T second) { return first + second; }

auto add_lambda = [](auto first, auto second) { return first + second; };

int main()
{
    // OK
    std::cout << std::apply(add, std::make_pair(1, 2)) << '\n';

   // Error: can't deduce the function type
   // std::cout << std::apply(add_generic, std::make_pair(2.0f, 3.0f)) << '\n'; 

   // OK
   std::cout << std::apply(add_lambda, std::make_pair(2.0f, 3.0f)) << '\n'; 
}

我得到这个错误:

toy.cpp: In function ‘int main()’:
toy.cpp:15:18: error: ‘apply’ is not a member of ‘std’
     std::cout << std::apply(add, std::make_pair(1, 2)) << '\n';
                  ^~~
toy.cpp:21:18: error: ‘apply’ is not a member of ‘std’
     std::cout << std::apply(add_lambda, std::make_pair(2.0f, 3.0f)) << '\n';

我用这个命令编译: g++ -std=c++17 toy.cpp -o toy

这是我的 g++ 版本:

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 6.3.0-18+deb9u1' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 6.3.0 20170516 (Debian 6.3.0-18+deb9u1)

我也试过用 clang，但我有同样的错误。我的 clang 版本是:

clang version 8.0.0 
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /etc/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6.3.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.3.0
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.3.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64

问题是什么？

Answer 1

你必须将你的 G++ 编译器升级到至少 7 版本，或者使用

#include <experimental/tuple>

然后调用

std::cout << std::experimental::apply(add, std::make_pair(1, 2)) << '\n';

Live