C++ function argument safety

Question

In a function that takes several arguments of the same type, how can we guarantee that the caller doesn't mess up the ordering?

For example

void allocate_things(int num_buffers, int pages_per_buffer, int default_value ... 

and later

// uhmm.. lets see which was which uhh.. allocate_things(40,22,80,... 

Answer 1

A typical solution is to put the parameters in a structure, with named fields.

AllocateParams p; p.num_buffers = 1; p.pages_per_buffer = 10; p.default_value = 93; allocate_things(p); 

You don't have to use fields, of course. You can use member functions or whatever you like.

Answer 2

If you have a C++11 compiler, you could use user-defined literals in combination with user-defined types. Here is a naive approach:

struct num_buffers_t {     constexpr num_buffers_t(int n) : n(n) {}  // constexpr constructor requires C++14     int n; };  struct pages_per_buffer_t {     constexpr pages_per_buffer_t(int n) : n(n) {}     int n; };  constexpr num_buffers_t operator"" _buffers(unsigned long long int n) {     return num_buffers_t(n); }  constexpr pages_per_buffer_t operator"" _pages_per_buffer(unsigned long long int n) {     return pages_per_buffer_t(n); }  void allocate_things(num_buffers_t num_buffers, pages_per_buffer_t pages_per_buffer) {     // do stuff... }  template <typename S, typename T> void allocate_things(S, T) = delete; // forbid calling with other types, eg. integer literals  int main() {     // now we see which is which ...     allocate_things(40_buffers, 22_pages_per_buffer);      // the following does not compile (see the 'deleted' function):     // allocate_things(40, 22);     // allocate_things(40, 22_pages_per_buffer);     // allocate_things(22_pages_per_buffer, 40_buffers); }