readability-string-compare

Finds string comparisons using the compare method.

A common mistake is to use the string's compare method instead of using the equality or inequality operators. The compare method is intended for sorting functions and thus returns a negative number, a positive number or zero depending on the lexicographical relationship between the strings compared. If an equality or inequality check can suffice, that is recommended. This is recommended to avoid the risk of incorrect interpretation of the return value and to simplify the code. The string equality and inequality operators can also be faster than the compare method due to early termination.

Examples:

std::string str1{"a"};
std::string str2{"b"};

// use str1 != str2 instead.
if (str1.compare(str2)) {
}

// use str1 == str2 instead.
if (!str1.compare(str2)) {
}

// use str1 == str2 instead.
if (str1.compare(str2) == 0) {
}

// use str1 != str2 instead.
if (str1.compare(str2) != 0) {
}

// use str1 == str2 instead.
if (0 == str1.compare(str2)) {
}

// use str1 != str2 instead.
if (0 != str1.compare(str2)) {
}

// Use str1 == "foo" instead.
if (str1.compare("foo") == 0) {
}

The above code examples shows the list of if-statements that this check will give a warning for. All of them uses compare to check if equality or inequality of two strings instead of using the correct operators.