Question

The question is about modeling infinity in C++ for the double data type. I need it in a header file, so we cannot use functions like numeric_limits.

Is there a defined constant that represents the largest value?

Answer 1

floating point numbers(such as doubles) can actually hold positive and negative infinity. The constant INFINITY should be in your math.h header.

Went standard diving and found the text:

4 The macro INFINITY expands to a constant expression of type float
representing positive or unsigned infinity, if available; else to a
positive constant of type float that overflows at translation time.

In Section 7.12 Mathematics <math.h>

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Then of course you have the helper function isinf to test for infinity(which is also in math.h).

7.12.3.3 The isinf macro

int isinf(real-floating x);

Description: The isinf macro determines whether its argument value is an infinity (positive
or negative). First, an argument represented in a format wider than
its semantic type is converted to its semantic type. Then
determination is based on the type of the argument.

Returns: The
isinf macro returns a nonzero value if and only if its argument has an
infinite value.