literal class compile error with constexpr constructor and function (differ vc, g++)

Question

#include <iostream>
#include <string>
using namespace std;

class A {
public:
    constexpr A() {}
    constexpr int area() {
        return 12;
    }
private:
//  constexpr int h = 3;
//  constexpr int w = 4;
};
int main()
{
    constexpr A a;
    constexpr int j = a.area();
    cout << j << endl;

}

Why the code above can't compile with MSVC compiler while works with g++? Isn't MSVC not as strict as other compilers? The difference results between MSVC and g++ is sometimes confusing. Which compiler should I rely on, any tips btw?

Your image of text isn't very helpful. It can't be read aloud or copied into an editor, and it doesn't index very well, meaning that other users with the same problem are less likely to find the answer here. Please edit your post to incorporate the relevant text directly (preferably using copy+paste to avoid transcription errors). — Toby Speight
– Toby Speight, Commented May 3, 2018 at 14:25
I thought you said there were error messages in them? If so, they should be in the question (as text) instead of the images. — Toby Speight
– Toby Speight, Commented May 4, 2018 at 7:30

Ivaylo Valchev · Accepted Answer · 2018-05-03 03:58:29Z

4

The problem is that a constexpr object implies const, which means you cannot call area as it is a non-const function. Mark area as const and that's it.

Alternatively, making a non-const will allow you to keep area non-const, which whilst odd, it's valid C++.

EDIT. Perhaps you are using C++14 or above. Your impression that a constexpr function implies const is a C++11 feature that was changed in later standards.

edited May 3, 2018 at 3:58

answered May 3, 2018 at 3:50

Ivaylo Valchev

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6 Comments

N00byEdge Over a year ago

Sure, but making a non-const would make the constructor etc not be run at compile time. Kind of feels like the point of this.

Rick Over a year ago

but constexpr memeber function is implictly const.

Ivaylo Valchev Over a year ago

@Rick No, it does not. constexpr int area() const notice the second const.

Rick Over a year ago

Yes, I am reading C++ Primer and it's C++11 I think. But I still don't understand why I can compiler this with g++ and I think my code is right.

Ivaylo Valchev Over a year ago

@Rick When compiling with g++ use the command -std=c++11 to compile in C++11 mode. godbolt.org/g/DZ5x2Q

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