Portable Way to Define Maximum Value for Unsigned Integer

I found this technique by chance.

If you take a look at man page of std::basic_string::npos_ref, you will fine detail of a technique to define a maximum value for unsigned integer type in which it is size_type.

Basically you assign -1 to unsigned integer type variable to assign a maximum value to it. This is due to signed-to-unsigned implicit conversion, and it is portable way to do it.

Consider the following test C++ program I’ve conducted

#include <iostream>
#include <limits>

int main()
{
    std::cout << "sizeof(unsigned int) = " << sizeof(unsigned int) << std::endl;
    std::cout << "maximum value of unsigned int (from std::numberic_limits<unsigned int>::max()): " << std::numeric_limits<unsigned int>::max() << std::endl;
    unsigned int maxValue = -1;
    std::cout << "maximum value of unsigned int (from -1 technique): " << maxValue << std::endl;
    std::cout << "equal?: " << std::boolalpha << (maxValue == std::numeric_limits<unsigned int>::max()) << std::endl;
    return 0;
}

You can get minimum and maximum value of certain integer type via std::numeric_limits. So above program get such value from it, and validate whether maximum value is equal to assigning -1 technique.

It is equal.

The following is output of the program

sizeof(unsigned int) = 4
maximum value of unsigned int (from std::numberic_limits<unsigned int>::max()): 4294967295
maximum value of unsigned int (from -1 technique): 4294967295
equal?: true

For reference, source code is hosted here.

First published on Sep, 29, 2019

Written by Wasin Thonkaew
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