c++ - Why isn't this 'for' loop valid? -

consider:

int ia[3][4]; (auto row : ia)         (auto col : row) 

the first for iterates through ia, elements arrays of size 4. because row not reference, when compiler initializes row convert each array element (like other object of array type) pointer array’s first element. result, in loop type of row int*.

i not sure understand how auto works, if can assume automatically gives type row based on ia array members type, don't understand why kind of for, row not reference, not valid. why going happen? "pointer array’s first element", because of what?

the problem row int * , not int[4] 1 expect because arrays decay pointers , there no automatic way know how many elements pointer points to.

to around problem std::array has been added works expected:

#include <array>  int main() {     std::array<std::array<int, 4>, 3> ia;     (auto &row : ia){         (auto &col : row){             col = 0;         }     } } 

note & before row , col indicate want reference , not copy of rows , columns, otherwise setting col 0 have no effect on ia.