Question

I have const char* FilePathName which looks like this: C:\ImportantFile.hex
And an int id = 12345;
I need to define a new const char* FilePathName_ID that would append the id with an underscore to the original FilePathName to look like this: C:\ImportantFile_12345.hex
I have looked at this but its different as I am using const char* which gives me the error cannot convert from 'const char * ' to 'char'and need an underscore.
I need to end up with a const char*
EDIT I need to keep the file extension.

Answer 1

You need to create a new std::string object or a null-terminated byte string. One easy way is this:

std::string append_number(std::string const& x, unsigned int num, char sep = '_') {
    std::stringstream s;
    s << strip_extension(x) << sep << num;
    return s.str();
}

You can pass a string literal to the above function seamlessly.

Update: I notice that you probably also need to strip the extension:

std::string strip_extension(std::string x, char ext_sep = '.') {
     return x.substr(0, x.find_last_of(ext_sep));
}

std::string get_extension(std::string const& x, char ext_sep = '.') {
     return x.substr(x.find_last_of(ext_sep) + 1); // no error checking 
}

See updated definition of append_number.

Update 2: Try the following program:

#include <string>
#include <iostream>
#include <sstream>

std::string strip_extension(std::string const& x, char ext_sep = '.') {
     return x.substr(0, x.find_last_of(ext_sep));
}

std::string append_number(std::string const& x, unsigned int num, char sep = '_') {
        std::stringstream s;
        s << strip_extension(x) << sep << num << '.' << get_extension(x);
        return s.str();
    }

int main() {
  std::cout << append_number("file.hex", 45) << std::endl;
}

The output should be:

file_45.hex