鼻子塌的男生性功能:这个程序怎么不能执行？string不能用

//=====================================
//f0205.cpp
//倒三角形
//=====================================
#include <iostream>
using namespace std;
//-------------------------------------
int main(){
for (int n=1; n<=10; ++n)
cout <<string(n,' ')+string(21-2*n,'M')+"\n";
}//====================================
编译的时候系统说有两个问题，用的VC++创天中文。
--------------------Configuration: f021011 - Win32 Debug--------------------
Compiling...
f021011.cpp
E:\3 学习\C++\f021011.cpp(10) : error C2784: 'class std::reverse_iterator<_RI,_Ty,_Rt,_Pt,_D> __cdecl std::operator +(_D,const class std::reverse_iterator<_RI,_Ty,_Rt,_Pt,_D> &)' : could not deduce template argument for '' from 'class std::basic_str
ing<char,struct std::char_traits<char>,class std::allocator<char> >'
E:\3 学习\C++\f021011.cpp(10) : error C2676: binary '+' : 'class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >' does not define this operator or a conversion to a type acceptable to the predefined operator
E:\3 学习\C++\f021011.cpp(11) : warning C4508: 'main' : function should return a value; 'void' return type assumed
Error executing cl.exe.

f021011.exe - 2 error(s), 1 warning(s)