Why can't C++ deduce template type from assignment?

Question

int x = fromString(\"test\") :could not deduce template argument for \'ValueType\'

int x = fromString(\"test\") : works

Answer 1

You can't deduce based on the return type. You can, however, implement a workaround with similar syntax, using the overloaded cast operator:

#include 
#include 
#include 
using namespace std;

class FromString{
private:
    string m_data;
public:
    FromString(const char*data) : m_data(data) {} 

    template
    operator T(){
        T t;
        stringstream ss(m_data);
        ss >> t;
        return t;
    }

};

template<> FromString::operator bool(){
    return (m_data!="false"); //stupid example
}

int main(){

    int ans = FromString("42");    
    bool t = FromString("true");
    bool f = FromString("false");

    cout << ans << " " << t << " " << f << endl;

    return 0;
}

Output:

42 1 0