std::memcmp is a simple function that performs the simple task of comparing the contents of two objects. The result is an integer, which is either less than zero or more than zero. The function can be used for a variety of types, including scalar and struct types, enum and trait, and fn and macro.
Unlike the std::equal function, which is used to compare the contents of a single object, std::memcmp compares the contents of multiple objects. It has a trivially non-deleted destructor. The function is useful when there are redundant copies of objects in the same memory location. This function is also useful in cases where the object being compared is of a narrow character type, which allows modifications to be performed solely based on the bits that make up the object’s value representation.
std::memcmp can be used in conjunction with other functions that are important for memory management, including memset and memmove. Using these functions in conjunction with each other may speed up your program. Using these functions together may also give you unexpected results. In particular, you may have a hard time determining which object is the most important to your program.
The memcmp function does not handle iterator objects, so it should be used for those types of objects that are trivially copyable. It is also not useful for objects of a non-trivially copyable type. In the real world, however, you may be able to get away with using std::memcmp to compare objects of a narrow character type. If you are not sure whether your object is of a narrow character type, a quick prefix search on the object’s name will tell you if it is.
The std::memcmp function is not the only comparison function that can be used to show off the best of the C++ programming language. std::equal is a similar function, but it can handle pointers. std::equal performs the same comparison as memcmp, but it also includes additional functions. Using std::equal may be preferable to memcmp if you are comparing an array, or if you are comparing objects of the same type.
Using the std::memcmp and std::memset functions together may cause undefined behavior. Depending on the type of object being compared, std::memset may clear the internal state of the object, which will result in an undefined result. Using std::memcmp may also cause you to encounter a problem if you have to dispatch a virtual method to a virtual method that is implemented by a class that is part of the object’s value representation. The std::memcmp method is also the shortest loop in the C++ standard, and can be easily misunderstood. Using the std::equal method may also give you more control over the outcome of your comparison.
The std::memcmp functions also show off the C++ Standard’s bit-field data member and padding capabilities. When using this function, be sure to keep in mind that the padding bits may be illuminating. If you are trying to compare two objects with bits in common, be sure to avoid padding bits that may be irrelevant.