Dtjytyk

I am trying to implement in a C++ class a constexpr member function which returns a template parameter. The code is supposed to be c++11 compatible. However I encounter compilation issues when the templated class also contains STL containers as data members such as std::vector (which are untouched by the constexpr member function).

A minimal example is given by the following code:

#include <vector>
#include <iostream>
#include <array>


template<size_t n>
struct A 


 constexpr size_t dimensions() const
 
 return n;
 
private:
 std::vector<double> a;
;


int main(int argc,char ** argv)

 auto a=A<3>();
 std::array<double,a.dimensions()> arr;

The code compiles correctly with the commands

g++ -std=c++14 -O3 quickTest.cpp -o test -Wall

clang++ -std=c++11 -O3 quickTest.cpp -o test -Wall

but fails when I use

g++ -std=c++11 -O3 quickTest.cpp -o test -Wall

with the error

quickTest.cpp:22:33: error: call to non-‘constexpr’ function ‘size_t A<n>::dimensions() const [with long unsigned int n = 3; size_t = long unsigned int]’
 std::array<double,a.dimensions()> arr;
 ~~~~~~~~~~~~^~
quickTest.cpp:10:20: note: ‘size_t A<n>::dimensions() const [with long unsigned int n = 3; size_t = long unsigned int]’ is not usable as a ‘constexpr’ function because:
 constexpr size_t dimensions() const
 ^~~~~~~~~~
quickTest.cpp:22:33: error: call to non-‘constexpr’ function ‘size_t A<n>::dimensions() const [with long unsigned int n = 3; size_t = long unsigned int]’
 std::array<double,a.dimensions()> arr;
 ~~~~~~~~~~~~^~
quickTest.cpp:22:33: note: in template argument for type ‘long unsigned int’

Why does the code not compile with gcc -std=c++11 but does compile with clang++ -std=c++11 ?
How could one make this code snippet work with older versions of gcc which many not support c++14/17, but only c++11 ?

I am using gcc 8.1.1 and clang 6.0.1

C++11 had a rule [dcl.constexpr]/8:

... The class of which that function is a member shall be a literal type ([basic.types]).

struct A is not a literal type because of the vector, hence its non-static member functions cannot be constexpr.

So GCC is right to reject the code in C++11 mode.

C++14 removed that restriction.

The solution for C++11 is to declare dimensions() static:

 static constexpr size_t dimensions()
 
 return n;
 

Live demo