How to set a const int to maximum in C++?

Question

I have a static const member and would like to set it to the maximum integer. I'm trying the following:

const static int MY_VALUE = std::numeric_limits<int>::max();

But get the following error:

error: in-class initializer for static data member is not a constant expression

Is there any solution to this? How can a function not return a constant expression?

EDIT: Adding -std=c++11 fixed the issue. My roommate tells me that the compiler (pre C++11) isn't smart enough to decide that std::numeric_limits::max() doesn't mutate anything else, and so is not considered constant. Is that possibly the reason for this error?

Answer 1

A constant must be initialized from a constant expression (an expression evaluable at compile-time).

In C++03, the set of constant operations you could build constant expressions from was extremely tight. Only bare integrals and mathematical operations on those.

In order to use a user-defined function in a constant expression, you need:

• C++11, or greater
• said function to be marked constexpr

This is why adding the -std=c++11 flag to Clang helped: it allowed constexpr and "switched" to the improved Standard Library implementation which uses constexpr for std::numeric_limits<T>::max().

Note: if you use a more recent version of Clang, C++11 will be the default and no flag will be necessary to allow constexpr.

Answer 2

Like this:

#include <climits>
const static int MY_VALUE = INT_MAX;

Answer 3

If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions.

The member shall still be defined in a name- space scope if it is used in the program and the namespace scope definition shall not contain an initializer.

numeric_limits max() isn't an integral constant, that's compile time constant.