making a new char* out of two old char*s - c++

Question

I am open to using intermediary C++ code, though C code is the preference.
I have code like the following:
char *fileName1 = "graph";
char *extension1 = ".eps";
I want to create a new char* variable called fileName1WithExtension1 which would correspond to "graph.eps", formed from the two char* variables given above. How can this be done?

Answer 1

If you use C++, have those as std::string strings:
std::string fileName1 = "graph";
std::string extension1 = ".eps";
And then simply
std::string fileName1WithExtension1 = filename1 + extension1;
If you then need to pass that to a C library function expecting a C string, get char pointer with fileName1WithExtension1.c_str()
There really is no reason to muck about with plain C strings in C++ code. It is so error-prone and tedious, that it should be actively avoided.

Answer 2

char *new_string;
new_string = malloc(strlen(fileName1) + strlen(extension1) + 1);
sprintf(new_string, "%s%s", fileName1, extension1);
...
free(new_string)

Answer 3

You can use the asprintf() function
char *buffer;
asprintf (&buffer, "%s%s", fileName1, extension1);
When the buffer variable become useless in your code, you have to free the memory allocated for buffer by asprintf with
free(buffer);

Answer 4

char *joined;
joined = (char*)malloc(strlen(fileName1) + strlen(extension1) + 1);
strcpy(joined, fileName1)
strcat(joined, extension1)
For a small performance increase, if the compiler is smart enough at optimizing, change the last line to
strcpy(joined+strlen(fileName1), extension1)
Even better, store the length of fileName1 in a variable the first time you determine it, and use it in the final strcpy().

Answer 5

If you want to go REALLY low-level, with ugly loops and such, you can do this: (Tested, it compiles and gives the expected and desired results)
char* filename1 = "graph";
char* extension1 = ".eps";
char* filename1WithExtension1 = combine(filename1, extension1);
where:
char* combine(char* str1, char* str2)
{
int str1len = 0, str2len = 0;
while(str1[str1len] != '\0') {
str1len++;
}
while(str2[str2len] != '\0') {
str2len++;
}
int outputlen = str1len + str2len + 1;
char* output = new char[outputlen];
for(int i = 0; i < str1len; i++)
{
output[i] = str1[i];
}
for(int i = str1len; i < outputlen; i++)
{
output[i] = str2[i - str1len];
}
return output;
}

Answer 6

I did a bit of C brush-up for fun, here's an alternative (C90 and C++ compatible code) for joining an array of C strings with separator. It should be very efficient with any decently optimizing compiler, too:
#include <string.h>
#include <stdlib.h>
/* **parts are strings to join, a NULL-terminated array of char*
* sep is separator string, use "" for no separator, must not be NULL
* returns malloc-allocated buffer which must be freed
* if len_out!=NULL, sets *len_out to strlen of result string */
char *astrjoin(int *len_out, const char *sep, char **parts) {
int part_count;
int parts_total_len = 0;
for(part_count = 0; parts[part_count]; ++part_count) {
parts_total_len += strlen(parts[part_count]);
}
if (part_count > 0) {
int malloc_size = (part_count - 1) * strlen(sep) + parts_total_len + 1;
char *result = (char*)malloc(malloc_size);
char *dest = result;
for(;;) {
const char *src;
for(src=*parts; *src; ++src) *dest++ = *src;
if (!*++parts) break;
for(src=sep ; *src; ++src) *dest++ = *src;
}
*dest = 0;
if (len_out) *len_out = malloc_size - 1;
return result;
} else {
if (len_out) *len_out = 0;
return strdup("");
}
}
Example usage:
int main(int argc, char *argv[]) {
/* argv is NULL-terminated array of char pointers */
char *commandline = astrjoin(NULL, " ", argv);
printf("argc: %d\nargv: %s\n", argc, commandline);
free(commandline);
return 0;
}
To call that in context of your question, it could be something like:
char *tmparr[] = { fileName1, exteansion1, NULL };
char *fileName1WithExtension1 = astrjoin(NULL, "", tmparr);
It would be trivial to create a version with sep and/or len_out dropped, or a version supporting "varargs", with signature looking something like:
char *astrjoin_va(int *len_out, const char *sep, ...);
Which would be nicer to call in context of your question:
char *fileName1WithExtension1 = astrjoin_va(NULL, "", fileName1, extension1, NULL);