Question

I have a function for writing ppm files (a picture format) to disk. It takes the filename as a char* array. In my main function, I put together a filename using a stringstream and the << operator. Then, I want to pass the results of this to my ppm function. I’ve seen this discussed elsewhere, often with very convoluted looking methods (many in-between conversion steps).

What I’ve done is shown in the code below, and the tricky part that others usually do in many steps with temp variables is (char*) (PPM_file_name.str().data()). What this accomplishes is to extract the string from stringstream PPM_file_name with .str(), then get the pointer to its actual contents with .data() (this is a const char*), then cast that to a regular (char*). More complete example below.

I’ve found the following to work fine so far, but it makes me uneasy because usually when other people have done something in a seemingly more convoluted way, it’s because that’s a safer way to do it. So, can anyone tell me if what I’m doing here is safe and also how portable is it?

Thanks.

#include <iostream>
#include <sstream>
#include <stdio.h>
#include <string>
using namespace std;

int main(int argc, char *argv[]){

    // String stream to hold the file name so I can create it from a series of other variable
    stringstream PPM_file_name; 

    // ... a bunch of other code where int ccd_num and string cur_id_str are created and initialized

    // Assemble the file name
    PPM_file_name << "ccd" << ccd_num << "_" << cur_id_str << ".ppm";

    // From PPM_file_name, extract its string, then the const char* pointer to that string's data, then cast that to char*
    write_ppm((char*)(PPM_file_name.str().data()),"ladybug_vidcapture.cpp",rgb_images[ccd_num],width,height);                   

    return 0;
}

Answer 1

This looks like a typical case of someone not writing const-correct code and it having the knock-on effect. You have several choices:

• If write_ppm is under your control, or the control of anyone you know, get them to make it const corrct

• If it is not, and you can guarantee it never changes the filename then const_cast

• If you cannot guarantee that, copy your string into a std::vector plus the null terminator and pass &vec[0] (where vec represents the name of your vector variable)