Question

I want to refactor some printf/sprintf/fprintf statements into ostream/sstream/fstream statements. The code in question pretty-prints a series of integers and floating-point numbers, using whitespace padding and fixed numbers of decimal points.

It seems to me that this would be a good candidate for a Martin Fowler style writeup of a safe, step-by-step refactorings, with important gotchas noted. The first step, of course, is to get the legacy code into a test harness, which I have done.

What slow and careful steps can I take to perform this refactoring?

Answer 1

Basic mechanics of the conversion:

• Convert each printf-style clause %w.pf or %w.pe, where w is the field width and p is the number of digits of precision, into << setw(w) << setprecision(p) << fixed.
• Convert each printf-style clause %wd or %wi, where w is the field width, into << setw(w).
• Convert "\n" to endl where appropriate.

Process for printf:

• Create a char[] (let’s call it text) with enough total width.
• Convert the printf(...) to sprintf(text, ...), and use cout << text to actually print the text.
• Complete using the common instructions.

Process for fprintf:

• Same as printf, but use the appropriate fstream instead of cout.
  • If you already have an opened C-style FILE object that you do not want to refactor at this time, it gets a little sticky (but can be done).
• Complete using the common instructions.

Process for sprintf:

• If the string being written to is only used to output to a stream in the current context, refer to one of the two refactorings above.
  • Otherwise, begin by creating a stringstream and streaming the contents of the char[] you are writing to into that. If you are still intending to extract a char* from it, you can do std::stringstream::str().c_str().
• Complete using the common instructions.

Common instructions:

• Convert each clause one by one into C++-style.
• Remove *printf and char[] declarations as necessary when finished.
• Apply other refactorings, particularly “Extract Method” (Fowler, Refactoring) as necessary.