Question

I was running some benchmarks to find the most efficient way to write a huge array to a file in C++ (more than 1Go in ASCII).

So I compared std::ofstream with fprintf (see the switch I used below)

    case 0: {
        std::ofstream out(title, std::ios::out | std::ios::trunc);
        if (out) {
            ok = true;
            for (i=0; i<M; i++) {
                for (j=0; j<N; j++) {
                    out<<A[i][j]<<" ";
                }
                out<<"\n";
            }
            out.close();
        } else {
            std::cout<<"Error with file : "<<title<<"\n";
        }
        break;
    }
    case 1: {
        FILE *out = fopen(title.c_str(), "w");
        if (out!=NULL) {
            ok = true;
            for (i=0; i<M; i++) {
                for (j=0; j<N; j++) {
                    fprintf(out, "%d ", A[i][j]);
                }
                fprintf(out, "\n");
            }
            fclose(out);
        } else {
            std::cout<<"Error with file : "<<title<<"\n";
        }
        break;
    }

And my huge problem is that fprintf seems to be more thant 12x slower compared to std::ofstream. Do you have an idea of what is the origin of the problem in my code ? Or maybe std::ofstream is very optimized compared to fprintf ?

(and an other question : do you know another faster way to write a file)

Thank you very much

(detail : I was compiling with g++ -Wall -O3)

Answer 1

fprintf("%d" requires runtime parsing of the format string, once per integer. ostream& operator<<(ostream&, int) is resolved by the compiler, once per compilation.