std::memcpy

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:

• dest or src is a null pointer or invalid pointer.
• Copying takes place between objects that overlap.

[edit] Parameters

[edit] Return value

If there is a suitable created object, returns a pointer to it; otherwise returns dest.

[edit] 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.

[edit] 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();
}

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

[edit] Defect reports

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

[edit] See also