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CppMemcpy

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(C++) memcpy

Standard function [1] to copy raw memory.

std::memcpy(void * dest, const void * const src, const int nBytes)

Below is an example that shows the copying op a C-style string using both std::strcpy and std::memcpy. Note the first being easier as std::strcpy relies on a C-style string being null-terminated.

include <cstring>

include <cstdio>

include <cassert>


int main()
{
  char myArray[4] = {'\0'};
  {
    //Using std::strcpy
    std::strcpy(myArray,"yes");
    assert(std::strcmp(myArray,"yes")==0);
    std::printf("myArray: '%s'\n",myArray);
  }
  {
    //Using std::strcpy
    std::memcpy(myArray,"no\0",(std::size_t)sizeof(char) * 3);
    assert(std::strcmp(myArray,"no")==0);
    std::printf("myArray: '%s'\n",myArray);
  }
}

You can use it to copy a plain-old-data type to e.g. an array of char (which then can be copied to a full-fledged std::string):

include <iostream>

include <cstdio>

include <cstring>

include <cassert>


struct MyStruct
{
  char name[12]; // No terminator
  char date[8];  // No terminator
  char age[2];   // No terminator
};

int main()
{
  // Create a struct
  MyStruct m;
  // Create an array
  char a[sizeof(MyStruct) + 1] = { '\0' };
  {
    // Some compile-time asserts
    { const char temp[ (sizeof(m.name  ) == 12 ? 1 : 0) ]; }
    { const char temp[ (sizeof(m.date  ) ==  8 ? 1 : 0) ]; }
    { const char temp[ (sizeof(m.age   ) ==  2 ? 1 : 0) ]; }
    { const char temp[ (sizeof(MyStruct) == 22 ? 1 : 0) ]; }
  }
  // Initialize the struct
  std::memcpy(m.name,"Bilderbikkel",sizeof(m.name)); // No terminator
  std::memcpy(m.date,"20070509",sizeof(m.date));     // No terminator
  std::memcpy(m.age,"26",sizeof(m.age));             // No terminator
  // Copy the struct to the array
  std::memcpy(a,&m,sizeof(MyStruct));
  // Add a terminator to the array
  a[sizeof(MyStruct)] = '\0';
  // print the array
  std::printf("%s\n",a);
  std::cout << a << std::endl;
}

Don't use memcpy on non-plain-old-data types [2].

'memcpy' links

• C

Code links

• assert
• int
• main

Reference

• 1) C++ International Standard. ISO/IEC 14882. Second edition. Table 99
• 2) Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6, chapter 96: don't memcpy and memcmp non-POD's

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