Why doesn't C++ detect when out of range vector elements are accessed with the [ ] operators by default? -

i understand arrays primitive class , therefore not have built in methods detect out of range errors. however, vector class has built in function .at() detect these errors. using namespaces, can overload [ ] symbols act .at() function throwing error when value out of vector's range accessed. question this: why functionality not default in c++?

edit: below example in pseudocode (i believe - correct me if needed) of overloading vector operator [ ]:

item_type& operator[](size_t index) { // verify index legal. if (index < 0 || index >= num_items) {    throw std::out_of_range      ("index operator[] out of range"); }  return the_data[index] } 

i believe function can written user-defined namespace , reasonably easy implement. if true, why not default?

for that's cheap [], bounds checking adds significant overhead.

consider

int f1(const std::vector<int> & v, std:size_t s) { return v[s]; } 

this function translates just 3 lines of assembly:

    movq    (%rdi), %rax     movl    (%rax,%rsi,4), %eax     ret 

now consider bounds-checking version using at():

int f2(const std::vector<int> & v, std:size_t s) { return v.at(s); } 

this becomes

    movq    (%rdi), %rax     movq    8(%rdi), %rdx     subq    %rax, %rdx     sarq    $2, %rdx     cmpq    %rdx, %rsi     jae .l6     movl    (%rax,%rsi,4), %eax     ret .l6:     pushq   %rax     movl    $.lc1, %edi     xorl    %eax, %eax     call    std::__throw_out_of_range_fmt(char const*, ...) 

even in normal (non-throwing) code path, that's 8 lines of assembly - 3 times many.