c ++中基于范围的for循环

Question

It would seem that the "for each" style of syntax available in C++11 permits array iteration without knowledge of the actual size of the array (number of elements). I assume, since it is part of the new standard, that this is perfectly safe, even for C arrays. Typically, you must also separately know the size of a C array before manipulating it, but I want verification from anyone experienced with this new C++ technique that it works exactly as you'd expect:

extern float bunch[100];

for (float &f : bunch) {
  f += someNumber;
}

Is there anything I should know about non-obvious side effects or disadvantages to this technique? It doesn't show much in code I see, probably because most of the code was written before this was in the standard. Want to make sure its rare usage isn't because of some other reason not well-known.

Answer 1

There is nothing strange or unsafe about that usage. The array's size is known at compile time, so it can be used in the loop. This is an example of a template function that allows you to find out the length of an array:

template< class T, std::size_t N >
std::size_t length( const T (&)[N] )
{
  return N;
}

Foo f[5];
std::cout << length(f) << "\n";

This should make it clear that you cannot use this technique, or range based loops, on dynamically sized C-style arrays.

Of course, if you have range based loops then you should also have std::array (if not, you can probably get ti from std::tr1 or boost), so you can avoid the C-style array entirely:

extern std::array<float, 100> bunch;

for (auto &f : bunch) {
  f += someNumber;
}

Answer 2

It is perfectly safe to use a range-based for-loop with arrays. I suppose you're worried that you might accidentally use it on a pointer:

float* array = new float[100];
for (float& f : array) {
    // ...
}

Don't worry though. The compiler will produce an error in this case. So in cases where it's not safe, you'll get a compilation error anyway.

Answer 3

Arrays can be passed as references and have their type and length deduced.

#include <iostream>

template<typename T, size_t N>
void fun(T const (&arr)[N])
{
    for (std::size_t i = 0; i < N; ++i)
       std::cout << arr[i] << " ";
}

int main()
{
   int x[] = { 1, 2, 3 }; // no need to set length here
   fun(x); // 1 2 3, no need to specify type and length here
}