Use the auto keyword in C++ STL

Question

I have seen code which use vector,

vector<int>s; s.push_back(11); s.push_back(22); s.push_back(33); s.push_back(55); for (vector<int>::iterator it = s.begin(); it!=s.end(); it++) {     cout << *it << endl; } 

It is same as

for (auto it = s.begin(); it != s.end(); it++) {     cout << *it << endl; } 

How safe is in this case the use of the auto keyword? And what about if type of vector is float? string?

Answer 1

It's additional information, and isn't an answer.

In C++11 you can write:

for (auto& it : s) {     cout << it << endl; } 

instead of

for (auto it = s.begin(); it != s.end(); it++) {     cout << *it << endl; } 

It has the same meaning.

Update: See the @Alnitak's comment also.

Answer 2

The auto keyword is simply asking the compiler to deduce the type of the variable from the initialization.

Even a pre-C++0x compiler knows what the type of an (initialization) expression is, and more often than not, you can see that type in error messages.

#include <vector> #include <iostream> using namespace std;  int main() {     vector<int>s;     s.push_back(11);     s.push_back(22);     s.push_back(33);     s.push_back(55);     for (int it=s.begin();it!=s.end();it++){         cout<<*it<<endl;     } }  Line 12: error: cannot convert '__gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<int*, __gnu_norm::vector<int, std::allocator<int> > >, __gnu_debug_def::vector<int, std::allocator<int> > >' to 'int' in initialization 

The auto keyword simply allows you to take advantage of this knowledge - if you (compiler) know the right type, just choose for me!