How can I convert a std::string to int?

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梦毁少年i
梦毁少年i 2020-11-22 02:10

Just have a quick question. I\'ve looked around the internet quite a bit and I\'ve found a few solutions but none of them have worked yet. Looking at converting a string to

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  •  遇见更好的自我
    2020-11-22 02:47

    The possible options are described below:

    1. First option: sscanf()

        #include 
        #include 
    
            int i;
            float f;
            double d;
            std::string str;
    
            // string -> integer
            if(sscanf(str.c_str(), "%d", &i) != 1)
                // error management
    
            // string -> float
            if(sscanf(str.c_str(), "%f", &f) != 1)
                // error management
    
            // string -> double 
            if(sscanf(str.c_str(), "%lf", &d) != 1)
                // error management
    

    This is an error (also shown by cppcheck) because "scanf without field width limits can crash with huge input data on some versions of libc" (see here, and here).

    2. Second option: std::sto*()

        #include 
        #include 
    
            int i;
            float f;
            double d;
            std::string str;
    
            try {
                // string -> integer
                int i = std::stoi(str);
    
                // string -> float
                float f = std::stof(str);
    
                // string -> double 
                double d = std::stod(str);
            } catch (...) {
                // error management
            }   
    

    This solution is short and elegant, but it is available only on on C++11 compliant compilers.

    3. Third option: sstreams

        #include 
        #include 
    
            int i;
            float f;
            double d;
            std::string str;
    
            // string -> integer
            std::istringstream ( str ) >> i;
    
            // string -> float
            std::istringstream ( str ) >> f;
    
            // string -> double 
            std::istringstream ( str ) >> d;
    
            // error management ??
    

    However, with this solution is hard to distinguish between bad input (see here).

    4. Fourth option: Boost's lexical_cast

        #include 
        #include 
    
            std::string str;
    
            try {
                int i = boost::lexical_cast( str.c_str());
                float f = boost::lexical_cast( str.c_str());
                double d = boost::lexical_cast( str.c_str());
                } catch( boost::bad_lexical_cast const& ) {
                    // Error management
            }
    

    However, this is just a wrapper of sstream, and the documentation suggests to use sstream for better error management (see here).

    5. Fifth option: strto*()

    This solution is very long, due to error management, and it is described here. Since no function returns a plain int, a conversion is needed in case of integer (see here for how this conversion can be achieved).

    6. Sixth option: Qt

        #include 
        #include 
    
            bool ok;
            std::string;
    
            int i = QString::fromStdString(str).toInt(&ok);
            if (!ok)
                // Error management
    
            float f = QString::fromStdString(str).toFloat(&ok);
            if (!ok)
                // Error management 
    
            double d = QString::fromStdString(str).toDouble(&ok);
            if (!ok)
        // Error management     
    

    Conclusions

    Summing up, the best solution is C++11 std::stoi() or, as a second option, the use of Qt libraries. All other solutions are discouraged or buggy.

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