Use function objects to pass operations to algorithms

Reason

Function objects can carry more information through an interface than a "plain" pointer to function. In general, passing function objects gives better performance than passing pointers to functions.

Example

bool greater(double x, double y) { return x > y; }
sort(v, greater);                                    // pointer to function: potentially slow
sort(v, [](double x, double y) { return x > y; });   // function object
sort(v, std::greater{});                             // function object

bool greater_than_7(double x) { return x > 7; }
auto x = find_if(v, greater_than_7);                 // pointer to function: inflexible
auto y = find_if(v, [](double x) { return x > 7; }); // function object: carries the needed data
auto z = find_if(v, Greater_than<double>(7));        // function object: carries the needed data

You can, of course, generalize those functions using auto or concepts. For example:

auto y1 = find_if(v, [](totally_ordered auto x) { return x > 7; }); // require an ordered type
auto z1 = find_if(v, [](auto x) { return x > 7; });                 // hope that the type has a >

Note

Lambdas generate function objects.

Note

The performance argument depends on compiler and optimizer technology.

Enforcement

• Flag pointer to function template arguments.
• Flag pointers to functions passed as arguments to a template (risk of false positives).