什么接口会将cstrings，数组和其他类型复制到相同类型的实例？

Question

Not all instances of a type can be copied to another instance of the same type with an = sign.

For example, although it may work on ints :

int x = 0;
int y = 5;
x = y; //x is now: 5

---

It would not work on char arrays :

char x[32]="data to overwrite";
char y[32]="new data";
x = y; //incorrect

---

or other arrays :

int x[5] = {1,2,3,4,5};
int y[5] = {6,7,8,9,10};
x = y; //incorrect

or char*s :

char* x="data to overwrite";
char* y="new data";
x = y; //incorrect

How can I write an overloaded function that will allow me to do the following?

int x = 0;
int y = 5;
Copy(x,y); //x is now: 5

char x[32]="data to overwrite";
char y[32]="new data";
Copy(x,y); //x is now: "new data"

int x[5] = {1,2,3,4,5};
int y[5] = {6,7,8,9,10};
Copy(x,y); //x is now: {6,7,8,9,10}

char* x="data to overwrite";
char* y="new data";
Copy(x,y); //x is now: "new data"

*I'll assume that any abstract data types do the necessary work in their overloaded assignment operator(or from the shallow copy provided by the compiler)

---

Why do you need to do this?
In order to easier test portions of a legacy C code base, I'd like to generate some C++ wrappers around a few components. Due to the strange design of the C code, there is a lot of indirection that I would like to get rid of. As such, it would be a lot easier to copy variables into another instance using a simple Copy function instead of parsing out the types, and deciding how to make the appropriate copy into the other instance variable.

Answer 1

Here is a complete example, with cout s sprinkled in to show that the correct overloads are selected.

#include <algorithm>
#include <cstddef>
#include <iostream>
#include <ostream>

// default
template<class T>
void Assign(T& dst, const T& src)
{
  dst = src;
  std::cout << "assign (default)" << std::endl;
}

// arrays
template<class T1, std::size_t n>
void Assign(T1 (&dst)[n], const T1 (&src)[n])
{
  std::copy(src, src+n, dst);
  std::cout << "assign (array)" << std::endl;
}

// pointers
template<class T1>
void Assign(T1 *&dst, T1 *src)
{
  // DANGER: memory leaks/double frees
  // not exactly sure what is supposed to happen here
  // same as default for now...
  // ok as long as only string constants are passed around
  // (as is the case in the example)
  dst = src;
  std::cout << "assign (pointer)" << std::endl;
}

int main() {
  {
    int x = 0;
    int y = 5;
    Assign(x,y); //x is now: 5
  }

  {
    char x[32]="data to overwrite";
    char y[32]="new data";
    Assign(x,y); //x is now: "new data"
  }

  {
    int x[5] = {1,2,3,4,5};
    int y[5] = {6,7,8,9,10};
    Assign(x,y); //x is now: {6,7,8,9,10}
  }

  {
    const char* x="data to overwrite";
    const char* y="new data";
    Assign(x,y); //x is now: "new data"
  }
}

Output:

g++ -std=c++11 -g -Wall -O3 check.cc -o check && ./check
assign (default)
assign (array)
assign (array)
assign (pointer)

Answer 2

Here is my attempt on my own question:

  #include <algorithm>
  #include <cstring>

  //catch all
  template <typename T>
  void Copy(T &x, T y)
  {
      x = y;
  }

  //overload on static array copying
  //(T[N] = T[N])
  template <typename T, size_t N>
  void Copy(T(&x)[N], T(&y)[N])
  {
      std::copy(std::begin(y), std::end(y), std::begin(x));
  }

  //overload on copying of null terminated data
  //(char array = cstring)
  void Copy(char x[], const char y[])
  {
      //assumes x is >= to y
      //not sure if I can use strncpy somewhow
      strcpy(x, y); 
  }