Why std::size() is not a member of std in gcc 8.2.0

问题

I'm trying to teach myself some C++17.

Why is the compiler throwing an error for the below code snippet?

#include <iostream> 
#include <vector>
#include <iterator>

int main() 
{
    std::vector<int> v = { 3, 1, 4 };
    std::cout << std::size(v) << '\n'; 

    int a[] = { -5, 10, 15 };
    std::cout << std::size(a) << '\n';
}

The compiler throws the following error

manish@Manish-Tummala:~/c_files$ g++ 6.cpp -o - 6.out
6.cpp: In function ‘int main()’:
6.cpp:8:23: error: ‘size’ is not a member of ‘std’
     std::cout << std::size(v) << '\n';
                       ^~~~
6.cpp:8:23: note: suggested alternative: ‘size_t’
     std::cout << std::size(v) << '\n';
                       ^~~~
                       size_t
6.cpp:11:23: error: ‘size’ is not a member of ‘std’
     std::cout << std::size(a) << '\n';
                       ^~~~
6.cpp:11:23: note: suggested alternative: ‘size_t’
     std::cout << std::size(a) << '\n';
                       ^~~~
                       size_t

回答1:

For C++17 support in GCC, please refer to:

C++17 Support in GCC

For the time being, C++17 support is not enabled by default:

To enable C++17 support, add the command-line parameter -std=c++17 to your g++ command line. Or, to enable GNU extensions in addition to C++17 features, add -std=gnu++17.

At present, the C++17 ABI for GCC has not been finalized yet. This means that programs built today in C++17 mode may not be able to link against past or future binaries also compiled in C++17 mode (or crash at run time). A stable ABI ensures such interoperability across compiler versions.

Once the ABI is final, afuture version of GCC will enable C++17 mode by default.