C/C++: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' [-Wsign-compare]

Meaning

for (int i = 0; i < v.size(); ++i) triggers it because size() returns an unsigned size_t. With -1, the comparison -1 < 5u is false. Use size_t for indices, range-for, or cast deliberately after checking the sign.

Common causes

• Looping with int i against container.size()
• Comparing a possibly negative int with size_t/unsigned
• Mixing long and unsigned long variables
• Arithmetic that subtracts from size() and wraps below zero

Detailed fix by platform

Code examples

#include <vector>
#include <cstddef>

void run(const std::vector<int>& v) {
    // for (int i = 0; i < v.size(); ++i)      // warning: comparison of integer expressions of different signedness
    for (std::size_t i = 0; i < v.size(); ++i) {}
    for (const auto& x : v) { (void)x; }       // best: no index at all
}

How to diagnose

1. Types — Which operand is unsigned?
2. Range — Can the signed value be negative?
3. Loop — Can a range-for replace the index?

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