CODEWITHSUNDEEP
c++c++11constexpr
tai
tai

Reputation: 528

using constexpr double in namespaces

I'm currently getting into more C++11 stuff and jumped about constexpr. In one of my books it's said that you should use it for constants like π for example in this way:

#include <cmath>

// (...)

constexpr double PI = atan(1) * 4;

Now I wanted to put that in an own namespace, eg. MathC:

// config.h

#include <cmath>

namespace MathC {
    constexpr double PI = atan(1) * 4;
    // further declarations here
}

...but here IntelliSense says function call must have a constant value in a constant expression.

When I declare PI the following way, it works:

static const double PI = atan(1) * 4;

What is the actual reason the compiler doesn't seem to like constexpr but static const here? Shouldn't constexpr be eligible here, too, or is it all about the context here and constexprshouldn't be declared outside of functions?

Thank you.

Upvotes: 2

Views: 1031

Answers (1)

R Sahu
R Sahu

Reputation: 206717

What is the actual reason the compiler doesn't seem to like constexpr but static const here?

A constexpr must be evaluatable at compile time while static const does not need to be.

static const double PI = atan(1) * 4;

simply tells the compiler that PI may not be modified once it is initialized but it may be initialized at run time.

Upvotes: 5

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