NAME¶

std::memcpy - std::memcpy

Synopsis¶

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

If the objects are potentially-overlapping or not TriviallyCopyable, the behavior of
memcpy is not specified and may be 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¶

dest

Notes¶

std::memcpy may be used to implicitly create objects in the destination buffer.

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¶

// Run this code

#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}

See also¶

memmove moves one buffer to another
(function)
memset fills a buffer with a character
(function)
copies a certain amount of wide characters between two
wmemcpy non-overlapping arrays
(function)
copies characters
copy (public member function of
std::basic_string<CharT,Traits,Allocator>)
copy copies a range of elements to a new location
copy_if (function template)
(C++11)
copy_backward copies a range of elements in backwards order
(function template)
is_trivially_copyable checks if a type is trivially copyable
(C++11) (class template)
C documentation for
memcpy