Question

I would like to define inside a class a constant which value is the maximum possible int. Something like this:

class A
{
    ...
    static const int ERROR_VALUE = std::numeric_limits<int>::max();
    ...
}

This declaration fails to compile with the following message:

numeric.cpp:8: error: ‘std::numeric_limits::max()’ cannot appear in a constant-expression
numeric.cpp:8: error: a function call cannot appear in a constant-expression

I understand why this doesn’t work, but two things look weird to me:

1. It seems to me a natural decision to use the value in constant expressions. Why did the language designers decide to make max() a function thus not allowing this usage?

2. The spec claims in 18.2.1 that

   For all members declared static const in the numeric_limits template, specializations shall define these values in such a way that they are usable as integral constant expressions.

   Doesn’t it mean that I should be able to use it in my scenario and doesn’t it contradict the error message?

Thank you.

Answer 1

While the current standard lacks support here, for integral types Boost.IntegerTraits gives you the compile time constants const_min and const_max.

The problem arises from §9.4.2/4:

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.

Note that it adds:

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.

As others already mentioned numeric_limits min() and max() simply aren’t integral constant expressions, i.e. compile time constants.