020 string与数值转换

C++ 11 引入了多个函数，可以实现数值数据与标准库 string 之间的转换。

数值类型 转换为 string

to_string(val) 定义在 头文件 <string> 中，它是一组重载函数，返回值 val 的 string 表示。

    std::string to_string( int value );
    std::string to_string( long value );
    std::string to_string( long long value );
    std::string to_string( unsigned value );
    std::string to_string( unsigned long value );
    std::string to_string( unsigned long long value );
    std::string to_string( float value );
    std::string to_string( double value );
    std::string to_string( long double value );

注意：

1. 使用 std::to_string 转换浮点类型可能会产生意想不到的结果，因为返回的字符串中的有效数字可能为零，请参见下面的示例。
2. 返回值可能与默认情况下 std::cout 打印的值有很大的不同，请参见下面的示例。
3. std::to_string 依赖于当前语言环境进行格式化，因此来自多个线程的对 std::to_string 的并发调用可能导致调用的部分序列化。为此，c++ 17 提供 std::to_chars 作为一种与语言环境无关的高性能替代方案。

测试示例：

#include <QCoreApplication>
#include <iostream>
#include <string>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    double f = 23.43;
    double f2 = 1e-9;
    double f3 = 1e40;
    double f4 = 1e-40;
    double f5 = 123456789;
    std::string f_str = std::to_string(f);
    std::string f_str2 = std::to_string(f2); // Note: returns "0.000000"
    std::string f_str3 = std::to_string(f3); // Note: Does not return "1e+40".
    std::string f_str4 = std::to_string(f4); // Note: returns "0.000000"
    std::string f_str5 = std::to_string(f5);
    std::cout << "std::cout: " << f << '\n'
              << "to_string: " << f_str  << "\n\n"
              << "std::cout: " << f2 << '\n'
              << "to_string: " << f_str2 << "\n\n"
              << "std::cout: " << f3 << '\n'
              << "to_string: " << f_str3 << "\n\n"
              << "std::cout: " << f4 << '\n'
              << "to_string: " << f_str4 << "\n\n"
              << "std::cout: " << f5 << '\n'
              << "to_string: " << f_str5 << '\n';

    return a.exec();
}

输出结果：

std::cout: 23.43
to_string: 23.430000

std::cout: 1e-09
to_string: 0.000000

std::cout: 1e+40
to_string: 10000000000000000303786028427003666890752.000000

std::cout: 1e-40
to_string: 0.000000

std::cout: 1.23457e+08
to_string: 123456789.000000

string 转换为 数值类型

    int stoi( const std::string& str, std::size_t* pos = nullptr, int base = 10 );
    int stoi( const std::wstring& str, std::size_t* pos = nullptr, int base = 10 );
    long stol( const std::string& str, std::size_t* pos = nullptr, int base = 10 );
    long stol( const std::wstring& str, std::size_t* pos = nullptr, int base = 10 );
    long long stoll( const std::string& str, std::size_t* pos = nullptr, int base = 10 );
    long long stoll( const std::wstring& str, std::size_t* pos = nullptr, int base = 10 );
    unsigned long stoul( const std::string& str, std::size_t* pos = nullptr, int base = 10 );
    unsigned long stoul( const std::wstring& str, std::size_t* pos = nullptr, int base = 10 );
    unsigned long long stoull( const std::string& str, std::size_t* pos = nullptr, int base = 10 );
    unsigned long long stoull( const std::wstring& str, std::size_t* pos = nullptr, int base = 10 );

    float stof( const std::string& str, std::size_t* pos = nullptr );
    float stof( const std::wstring& str, std::size_t* pos = nullptr );
    double stod( const std::string& str, std::size_t* pos = nullptr );
    double stod( const std::wstring& str, std::size_t* pos = nullptr );
    long double stold( const std::string& str, std::size_t* pos = nullptr );
    long double stold( const std::wstring& str, std::size_t* pos = nullptr );

使用示例：

#include <QCoreApplication>
#include <iostream>
#include <string>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    std::string str1 = "45";
    std::string str2 = "3.14159";
    std::string str3 = "31337 with words";
    std::string str4 = "words and 2";

    int myint1 = std::stoi(str1);
    int myint2 = std::stoi(str2);
    int myint3 = std::stoi(str3);
    // error: 'std::invalid_argument'
    // int myint4 = std::stoi(str4);

    std::cout << "std::stoi(\"" << str1 << "\") is " << myint1 << '\n';
    std::cout << "std::stoi(\"" << str2 << "\") is " << myint2 << '\n';
    std::cout << "std::stoi(\"" << str3 << "\") is " << myint3 << '\n';
    //std::cout << "std::stoi(\"" << str4 << "\") is " << myint4 << '\n';

    return a.exec();
}

输出结果：

std::stoi("45") is 45
std::stoi("3.14159") is 3
std::stoi("31337 with words") is 31337