C++11中的to_string

C++11之前，标准库没有提供数字类型转字符串的函数，需要借助sprintf、stringstream等，现在C++11提供了std::to_string函数，可以直接使用了：

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string to_string (int val);
string to_string (long val);
string to_string (long long val);
string to_string (unsigned val);
string to_string (unsigned long val);
string to_string (unsigned long long val);
string to_string (float val);
string to_string (double val);
string to_string (long double val);

​除此之外，C++11标准库还引入了字符串转整数系列函数：

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int stoi (const string& str, size_t* idx = 0, int base = 10);
long stol (const string& str, size_t* idx = 0, int base = 10);
long long stoll (const string& str, size_t* idx = 0, int base = 10);
unsigned long stoul (const string& str, size_t* idx = 0, int base = 10);
unsigned long long stoull (const string& str, size_t* idx = 0, int base = 10);
double stod (const string& str, size_t* idx = 0);
float stof (const string& str, size_t* idx = 0);

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