Error with `std::reduce()` method in C++17 using GNU G++17 and Clang++ 17/20 compilers

Question

I encountered an error while using the std::reduce() method in C++17 with GNU G++17 and Clang++ 17/20 compilers. Despite including the <numeric> header, which should provide access to the reduce() method, the compiler reports that the method is not declared in the current scope. However, when I switch to the GNU G++20 compiler, the code compiles without any issues.

Code Sample:

#include <iostream>
#include <numeric>

int main() {
    int arr[] = {1, 2, 3, 4, 5};

    // Attempting to use std::reduce() method
    int sum = std::reduce(std::begin(arr), std::end(arr), 0);

    std::cout << "Sum: " << sum << std::endl;

    return 0;
}

Error Message (Compiler: GNU C++17 7.3.0):

program.cpp: In function 'int main()':
program.cpp:8:20: error: 'reduce' is not a member of 'std'
     int sum = std::reduce(std::begin(arr), std::end(arr), 0);
                    ^~~~~~

Error Message (Compiler: Clang++ 17/20):

p71.cpp:8:20: error: no member named 'reduce' in namespace 'std'
    int sum = std::reduce(std::begin(arr), std::end(arr), 0);
              ~~~~~^
1 error generated.

Only when I use the GNU C++20 compiler, the code compiles without any issues. However, I'm confused because I have already included the <numeric> header, and the reduce() method is introduced in the C++17 standard, which should be accessible. I'm wondering why the compiler fails to compile the code when using GNU G++17 and Clang++ 17/20 compilers.

I'm seeking clarification in understanding why this error is happening with certain compilers but not others. Can someone help me understand and resolve this issue? Are there any quirks or limitations associated with the reduce() method when using the Clang compiler? Any assistance would be greatly appreciated. Thank you in advance for your help!

Answer 1

The std::reduce function is part of the standard library starting with C++17. For GNU, it was implemented in gcc version 9.1. I can't determine the version for clang but it works for me back to at least clang-12.

There are two gating factors on whether you will be able to use std::reduce: 1) is it in the standard library for the compiler version that you are using and 2) do you have C++17 or later selected as the language standard.

Based on your description from above, you are using gcc version 7.3.0 which does not have std::reduce implemented in the corresponding standard library.

Saying that you are using the GNU C++17 or GNU C++20 compiler could correspond to a lot of different versions of gcc each of which with a different level of support for both language features and standard library features.