C++ syntax to name file via variable and text

Question

I have a program that takes in the name of a file as an argument (example: books.txt), runs, and then outputs the results to a new text file. I need to name the output file with an addendum (example: books_output.txt).

The method that I tried was

ofstream outputFile;
outputFile.open(argv[1] + "_output.txt", ofstream::out);

but this didn't compile. How can I make this work?

Answer 1

Your statement should look like this (as mentioned in my comment):

outputFile.open(std::string(argv[1]) + "_output.txt", ofstream::out);
             // ^^^^^^^^^^^^       ^

assumed argv[1] comes from the standard main() signature

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

argv[1] is a char* pointer and you can't concatenate char* pointers that way.

As some people bother regarding support of obsolete C++ standard versions, the std::ofstream::open() signatures of earlier versions didn't support a const std::string parameter directly, but only const char*. In case you have that situation your statement should look like

outputFile.open((std::string(argv[1]) + "_output.txt").c_str(), ofstream::out);