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

Revision as of 09:30, 21 April 2015

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 the objects are not Template:concept (e.g. scalars, arrays, C-compatible structs), the behavior is undefined unless the program does not depend on the effects of the destructor of the target object (which is not run by memcpy) and the lifetime of the target object (which is ended, but not started by memcpy) is started by some other means, such as placement-new.

Parameters

Return value

dest

Notes

std::memcpy is 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