在 C++ 中使用 bool/fail 将字符串转换为 int

2021-12-12 00:00:00 parsing c++ c++-faq

我有一个 std::string，它可以是一个字符串，也可以是一个值(例如 0).

I have a std::string which could be a string or could be a value (such as 0).

将 std::string 转换为具有失败能力的 int 的最佳或最简单的方法是什么?我想要 C# 的 Int32.TryParse 的 C++ 版本.

What is the best or easiest way to convert the std::string to int with the ability to fail? I want a C++ version of C#'s Int32.TryParse.

推荐答案

使用 boost::lexical_cast.如果无法完成转换，它将抛出异常.

Use boost::lexical_cast. If the cast cannot be done, it will throw an exception.

#include <boost/lexical_cast.hpp>
#include <iostream>
#include <string>

int main(void)
{
    std::string s;
    std::cin >> s;

    try
    {
        int i = boost::lexical_cast<int>(s);

        /* ... */
    }
    catch(...)
    {
        /* ... */
    }
}

<小时>

没有提升:

Without boost:

#include <iostream>
#include <sstream>
#include <string>

int main(void)
{
    std::string s;
    std::cin >> s;

    try
    {
        std::stringstream ss(s);

        int i;
        if ((ss >> i).fail() || !(ss >> std::ws).eof())
        {
            throw std::bad_cast();
        }

        /* ... */
    }
    catch(...)
    {
        /* ... */
    }
}

<小时>

虚假提升:

Faking boost:

#include <iostream>
#include <sstream>
#include <string>

template <typename T>
T lexical_cast(const std::string& s)
{
    std::stringstream ss(s);

    T result;
    if ((ss >> result).fail() || !(ss >> std::ws).eof())
    {
        throw std::bad_cast();
    }

    return result;
}

int main(void)
{
    std::string s;
    std::cin >> s;

    try
    {
        int i = lexical_cast<int>(s);

        /* ... */
    }
    catch(...)
    {
        /* ... */
    }
}

<小时>

如果你想要这些函数的无抛出版本，你必须捕获适当的异常(我不认为 boost::lexical_cast 提供无抛出版本)，比如这个:

If you want no-throw versions of these functions, you'll have to catch the appropriate exceptions (I don't think boost::lexical_cast provides a no-throw version), something like this:

#include <iostream>
#include <sstream>
#include <string>

template <typename T>
T lexical_cast(const std::string& s)
{
    std::stringstream ss(s);

    T result;
    if ((ss >> result).fail() || !(ss >> std::ws).eof())
    {
        throw std::bad_cast();
    }

    return result;
}

template <typename T>
bool lexical_cast(const std::string& s, T& t)
{
    try
    {
        // code-reuse! you could wrap
        // boost::lexical_cast up like
        // this as well
        t = lexical_cast<T>(s);

        return true;
    }
    catch (const std::bad_cast& e)
    {
        return false;
    }
}

int main(void)
{
    std::string s;
    std::cin >> s;

    int i;
    if (!lexical_cast(s, i))
    {
        std::cout << "Bad cast." << std::endl;
    }   
}

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