< cpp‎ | string‎ | byte
Defined in header <cstring>
void* memcpy( void* dest, const void* src, std::size_t count );

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 TriviallyCopyable (e.g. scalars, arrays, C-compatible structs), the behavior is undefined.


[edit] Parameters

dest - pointer to the memory location to copy to
src - pointer to the memory location to copy from
count - number of bytes to copy

[edit] Return value


[edit] 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

[edit] 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";


0x1.999999999999ap-4 is 3fb999999999999a as an std::int64_t

[edit] See also

moves one buffer to another
(function) [edit]
fills a buffer with a character
(function) [edit]
copies a range of elements to a new location
(function template) [edit]
copies a range of elements in backwards order
(function template) [edit]
checks if a type is trivially copyable
(class template) [edit]
C documentation for memcpy