Building / Merging character array pointers in C++ - c++

Question

I'm very new to C++ (coming from C#) and it's giving me puzzles :S
I have a very basic question about arrays and it's pointers.
So if I have the following code:
char * test1 = "com";
char * test2 = "ment";
I found similar code in some files already. I don't exactly understand how a string can fit in one character.. but ok...
However, how could I connect these arrays so that I get "comment" ?
I'm pretty sure this char * result = test1 + test2; would only increase the pointer which would then point to something in the memory, which I dont intend to use.
So is it possible to get an array like char array[] = {'c', 'o', 'm', 'm', 'e', 'n', 't'}; back from this?
or can I at least get a pointer which points to something like comment\NUL in the memory?

Answer 1

As you pointed out, pointer arithmetic can't solve this.
If you want to have a C-string as the result, allocating space for the whole new string is required, then copying over the characters, typically using strcat / strncat, but they are C-style string operations.
// Your C-strings
const char *test1 = "com";
const char *test2 = "ment";
// Dynamic allocation of memory for result string
char *result = new char[strlen(test1) + strlen(test2) + 1];
// Start with the empty string
*result = '\0';
// Concatenate both input strings (use strncat if you don't know
// for sure that they will fit into the result array!)
strcat(result, test1);
strcat(result, test2);
// (use result pointer)
// Free the memory after last usage
delete[] result;
In C++, you typically try to avoid them and use std::string instead. Even if you want a C-string as the result, you can use a temporary std::string for allocation and management of the required memory as well as for performing the concatenation:
// Your C-strings
const char *test1 = "com";
const char *test2 = "ment";
// Wrap in temporary C++ strings and concatenate:
std::string result = std::string(test1) + std::string(test2);
// Get the pointer (only valid as long as result is in scope!)
const char *ptr = result.c_str();
Furthermore, please note that you should not assign a string literal to a non-const char * pointer, use a const char* pointer instead. And try to avoid dealing with raw C-strings as long as possible; of course when you use C libraries you have to use them a lot.
Note also that above mentioned methods are performed at runtime; you can't get a compile-time solution for concatenating two string literals, even though the compiler could know what you want to have. I don't know your context, but maybe you only want to have a multi-line string literal, then simply drop the + and write "com" "ment".

Answer 2

A c style solution can be found in the following link:
http://www.cplusplus.com/forum/beginner/5681/:
int len = strlen(test1)+strlen(test2);
char* result = new char[len +1]; // +1 for null terminated string
snprintf(result,len +1, "%s%s",test1,test2);
result[len] = NULL;
// use result
delete(result);

Answer 3

You can utilize std::string:
#include <iostream>
int main() {
// Note: the character literals are const (non const is deprecated)!
const char * test1 = "com";
const char * test2 = "ment";
// This gives a compiler error (there is no way to add pointers)
// const char * concat = test1 + test2;
// A std::string has an overload for the operator +:
std::string comment = std::string(test1) + test2;
// The dynamically allocated string.
// Note: as soon as the comment string gets altered or destroyed the
// pointer s to the internal string data (may) become invalid.
const char* s = comment.c_str();
std::cout << s << '\n';
}