Question

I have a simple function that takes a char and return a string, in C it looks like;

char* get_string(char c) {
  switch(c) {
    case 'A': return "some string";
    Case 'B': return "some other string";
...

And it works fine, but then I wanted my code to work in C++, and the C++ compilers throws a gazillions “deprecated conversion from string constant to ‘char*’”. I understand the warning, but I’m not 100% sure what is the best way to implement the function so it will work, fast, on both C and C++. This function is being called a lot, it’s an important bottleneck, so it has to be fast. My best try is;

char* get_string(char c) {
  char* str = (char*)malloc(50);
  switch(c) {
    case 'A':
      sprintf(str, "some string");
      return str;
    Case 'B':
      sprintf(str, "some other string");
      return str;
...

Answer 1

Return a const char* instead of char*. The const in this context means “this pointer points to a constant pointee” – in other words, the caller cannot modify the returned string. This allows the compiler to place the strings in some memory location so the get_string() function can just return addresses to those strings. If the caller needs to modify the returned string, they can allocate their own buffer and copy it (preferably via std::string).

const char* get_string(char c) { 
  switch(c) { 
    case 'A': return "some string"; 
    case 'B': return "some other string"; 
...