在 C++ 中声明 INFINITY 的正确方法是什么？

Question

const int INFINITY = INT_MAX;

gives me the

expected an identifier

error. Could you please tell what am I missing? I tried to include <cmath> but it did not help.

Answer 1

The problem is that INFINITY is either a macro from the <cmath> header. This is expanded to an implementation-defined value by the preprocessor before the actual compilation. In the case of GCC (check with g++ -E ), the expression (__builtin_inff ()) takes the place of INFINITY , which clearly not a valid identifier.

A quick fix is to give your constant a different name, such that it is not reserved by the implementation (is not a name of a standard macro):

const int infinity = INT_MAX;

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But, when it comes to the title of the question:

What is the right way to declare the INFINITY in c++?

refer to this Q&A that suggests this C++ standard library equivalent:

#include <limits>

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

^{Note that integers do not have reserved infinity (Inf), or not a number (NaN) and operations that result in a value out of int 's range will (still) overflow, as opposed to operations with IEEE floating-point numbers, which according to this Q&A , don't overflow and result in Inf .}

Answer 2

INT_MAX is the C way. You should use the following in C++ :-

#include <limits>

int infinity = std::numeric_limits<int>::max();