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std::memcpy - cppreference.com

From cppreference.com

void* memcpy( void* dest, const void* src, std::size_t count );

Performs the following operations in order:

  1. Implicitly creates objects at dest.
  2. Copies count characters (as if of type unsigned char) from the object pointed to by src into the object pointed to by dest.

If any of the following conditions is satisfied, the behavior is undefined:

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

Return value

If there is a suitable created object, returns a pointer to it; otherwise returns 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 <cstdint>
#include <cstring>
#include <iostream>

int main()
{
    // simple usage
    char source[] = "once upon a daydream...", dest[4];
    std::memcpy(dest, source, sizeof dest);
    std::cout << "dest[4] = {";
    for (int n{}; char c : dest)
        std::cout << (n++ ? ", " : "") << '\'' << c << "'";
    std::cout << "};\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 a std::int64_t\n" << std::dec;
    
    // object creation in destination buffer
    struct S
    {
        int x{42};
        void print() const { std::cout << '{' << x << "}\n"; }
    } s;
    alignas(S) char buf[sizeof(S)];
    S* ps = new (buf) S; // placement new
    std::memcpy(ps, &s, sizeof s);
    ps->print();
}

Output: 

dest[4] = {'o', 'n', 'c', 'e'};
0x1.999999999999ap-4 is 3fb999999999999a as a std::int64_t
{42}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 4064 C++98 it was unclear whether the returned pointer points to a suitable created object made clear

See also

moves one buffer to another
(function) [edit]
fills a buffer with a character
(function) [edit]
copies a certain amount of wide characters between two non-overlapping arrays
(function) [edit]
copies characters
(public member function of std::basic_string<CharT,Traits,Allocator>) [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