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• Uniform initialization: you could use it to simplify some parts by not writing explicit types. In getColor, prefer:

   sf::Color getColor(int iterations) 
   {
   int r, g, b;
   // ...
   return {r, g, b};
   }
  

• Uniform initialization: you could use it to simplify some parts by not writing explicit types. In getColor, prefer:

   sf::Color getColor(int iterations) 
   {
   int r, g, b;
   // ...
   return {r, g, b};
   }
  

• Globals should generally be avoided;
• The maximum mandelbrot could default to MAX i.e.
  int mandelbrot(double startReal, double startImag, int maximum = ::MAX)

• Globals should generally be avoided;
• The maximum mandelbrot could default to MAX i.e.
  int mandelbrot(double startReal, double startImag, int maximum = ::MAX)
• zoom should have a maximum level and therefore you should ensure it's not broken (double-s are signed).

Other Details

• Globals should generally be avoided
• The maximum mandelbrot could default to MAX i.e.
  int mandelbrot(double startReal, double startImag, int maximum = ::MAX)

Performance

A minor optimization for getColor would try to reduce conditional assignments by giving r, g, b default values and so changing them only if necessary:

sf::Color getColor(int iterations) {
 int r = g = b = 0;
 if (iterations == -1) {
 // nothing to do
 } else if (iterations == 0) {
 r = 255;
 // g = 0;
 // b = 0;
 } else {
 // colour gradient: Red -> Blue -> Green -> Red -> Black
 // corresponding values: 0 -> 16 -> 32 -> 64 -> 127 (or -1)
 if (iterations < 16) {
 r = 16 * (16 - iterations);
 // g = 0;
 b = 16 * iterations - 1;
 } else if (iterations < 32) {
 // r = 0;
 g = 16 * (iterations - 16);
 b = 16 * (32 - iterations) - 1;
 } else if (iterations < 64) {
 r = 8 * (iterations - 32);
 g = 8 * (64 - iterations) - 1;
 // b = 0;
 } else { // range is 64 - 127
 r = 255 - (iterations - 64) * 4;
 // g = 0;
 // b = 0;
 }
 } 

Other Details

• Globals should generally be avoided;
• The maximum mandelbrot could default to MAX i.e.
  int mandelbrot(double startReal, double startImag, int maximum = ::MAX)