传递指向函数的指针而不复制它

Question

How do I pass data around my program without copying it every time?

Specifically, when calling sim(ohlc) I want to just pass the pointer reference, I don't want to copy the data to the function.

This is the program I made, but I'm not sure this is the best way to do it (specially when it comes to speed and memory usage).

I think I'm not passing the pointer to sim(ohlc) like I should, but if I try sim(&ohlc) I don't know how to change the sim function to accept that.

   struct ohlcS {
        vector<unsigned int> timestamp;
        vector<float> open;
        vector<float> high;
        vector<float> low;
        vector<float> close;
        vector<float> volume;
    } ;


ohlcS *read_csv(string file_name) {
    // open file and read stuff
    if (read_error)
        return NULL;
    static ohlcS ohlc;
    ohlc.timestamp.push_back(read_value);
    return &ohlc;
}

int sim(ohlcS* ohlc) {
    // do stuff
    return 1;
}


main() {
    ohlcS *ohlc = read_csv(input_file);
    results = sim(ohlc);
}

Answer 1

It's C++, use a reference. It's safe, since you return a static object.

static ohlc ohlc_not_found;

ohlc &read_csv(string file_name) {
    // open file and read stuff
    if(error_while_opening)
    {
        return ohlc_not_found;
    }
    static ohlc loc_ohlc;
    loc_ohlc.timestamp.push_back(read_value);
    return loc_ohlc;
}

int sim(const ohlc& par_ohlc) {
    // do stuff
    return 1;
}

....
 ohlc& var_ohlc = read_csv(input_file);
 if(var_ohlc == ohlc_not_found)
 {
      // error handling
      return;
 }
 results = sim(var_ohlc);

If you want to modify par_ohlc in sim, do not make it const.

and it's not recommended to use ohlc for both class and variable name :(

Answer 2

In line:

results = sim(ohlc);

you are passing ohlc pointer to sim function, no deep data copy is done, only 32bit pointer value is copied.

Answer 3

This pushes the address (32 bit value) onto the stack.

results = sim(ohlc);

Like:

  ; ...
  push eax ; addr of struct/class/whatever
  call function ; jump to function

  ; ...

function:
  push ebp
  mov ebp, esp
  mov eax, [ebp+8] ; ebp+8 is the 32 bit value you pushed before onto the stack
                   ; -> your pointer

Take a look at this and maybe that too.

Version 2

  ; ...
  push eax ; addr of struct/class/whatever
  jmp function ; jump to function
autolbl001:

  ; ...

function:
  push ebp
  mov ebp, esp
  mov eax, [ebp+8] ; ebp+8 is the 32 bit value you pushed before onto the stack
  ; ...
  jmp autolbl001