通过迭代器从字符串中删除空格到C ++ 11基于范围的for循环中的字符串

Question

I'm simply trying to delete all the whitespace from a string using C++11's range-based for loop; however, I keep getting std::out_of_range on basic_string::erase .

#include <iostream>
#include <string>
#include <typeinfo>

int main(){

  std::string str{"hello my name is sam"};

  //compiles, but throws an out_of_range exception
  for(auto i : str){
    std::cout << typeid(i).name();  //gcc outputs 'c' for 'char'
    if(isspace(i)){
      str.erase(i);
    }
  }
  std::cout << std::endl;

  //does not compile - "invalid type argument of unary '*' (have 'char')"
  for(auto i : str){
    if(isspace(*i)){
      str.erase(i);
    }
  }

  //works exactly as expected
  for(std::string::iterator i = begin(str); i != end(str); ++i){
    std::cout << typeid(*i).name();  //gcc outputs 'c' for 'char'
    if(isspace(*i)){
      str.erase(i);
    }
  }
  std::cout << std::endl;

}

So I'm wondering: what exactly is i in the first two loops? Why is it seemingly both a char (as verified by typeid ) and an iterator to a char (works with std::string::erase )? Why isn't it equivalent to the iterator in the last loop? It seems to me that they should function exactly the same.

Answer 1

The type of i in the range-based for loop is char , since the elements of a string are characters (more formally, std::string::value_type is an alias for char ).

The reason why it seems to work as an iterator when you pass it to erase() is that an overload of erase() exists that accepts an index and a count, but the latter has a default argument:

basic_string& erase( size_type index = 0, size_type count = npos );

And on your implementation char happens to be implicitly convertible to std::string::size_type . However, this is likely not doing what you expect.

To verify that i is not indeed an iterator, try dereferencing it and you will see the compiler screaming:

*i; // This will cause an error