Difference between revisions of "cpp/string/byte/memcpy"

Revision as of 14:36, 15 June 2018

Copies count bytes from the object pointed to by src to the object pointed to by dest. Both objects are reinterpreted as arrays of unsigned char.

If the objects overlap, the behavior is undefined.

If either dest or src is a null pointer, the behavior is undefined, even if count is zero.

If the objects are not TriviallyCopyable, the behavior of memcpy is not specified and may be undefined.

Parameters

Return value

dest

Notes

std::memcpy is meant to be the fastest library routine for memory-to-memory copy. It is usually more efficient than std::strcpy, which must scan the data it copies or std::memmove, which must take precautions to handle overlapping inputs.

Several C++ compilers transform suitable memory-copying loops to std::memcpy calls.

Where strict aliasing prohibits examining the same memory as values of two different types, std::memcpy may be used to convert the values.

Example

#include <iostream>
#include <cstdint>
#include <cstring>
 
int main()
{
    // simple usage
    char source[] = "once upon a midnight dreary...", dest[4];
    std::memcpy(dest, source, sizeof dest);
    for (char c : dest)
        std::cout << c << '\n';
 
    // reinterpreting
    double d = 0.1;
//  std::int64_t n = *reinterpret_cast<std::int64_t*>(&d); // aliasing violation
    std::int64_t n;
    std::memcpy(&n, &d, sizeof d); // OK
 
    std::cout << std::hexfloat << d << " is " << std::hex << n
              << " as an std::int64_t\n";
}

o
n
c
e
0x1.999999999999ap-4 is 3fb999999999999a as an std::int64_t

See also