什么时候应该在c ++中使用memcpy和memmove？

Question

I reflecting about memcpy.

I know that memcpy is for copy a variable to variable.

But it is not better to use: (for ex.)

int a = 5;
int b;

b = a;

Instead memcpy ?

or use std::move instead memmove ?

1. Memcpy and Memmove are not outdated/slowly functions (perhaps these functions are from C)?
2. If yes, what is the best way to replace these functions in C++11 standard ?

Answer 1

In order of decreasing importance, advantages are:

• memcpy and memmove are type agnostic, so they can be used to bypass strict aliasing restrictions.

• memcpy and memmove don't require alignment, although they probably run faster when data is aligned.

• memcpy may be faster, since it can copy multiple elements at once. With the std::is_trivially_copyable type trait, though, std::copy should do this also.

Of course, they have the disadvantage of only working with trivially copyable types, and only with pointers, not iterators. But they have definite uses in C++ code, for example when pulling unaligned data out of network packets or blocks of files, or implementing approximations to floating-point functions using bit tricks .

Answer 2

You are only considering one case. Nobody use memcpy and memmove for int variables.

They are basically highly optimized function to copy large blocks of data. They are the system functions. They directly deal with memory.