SparesMat::operator++() vs SparesMat::operator++(int)

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i guess, this is more an ordinary c++ problem, by convention,

• operator++() denotes the "postfix" increment (x++)
• operator++(int) denotes the "prefix" increment (++x)

the int arg in the latter is a dummy, you're not supposed to supply anything there (how would you even?)

the difference between them is the order of evaluation and increment, it's the same as with plain numbers:

int x = 3;
cerr << (x++); // prints 3, first eval, then increment
cerr << (x);   // prints 4 ;)
cerr << (++x); // prints 5, first increment, then eval