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

Question

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

int x = fromString<int>("test") : works fine as expected

So why does the compiler struggle here? I see it with all kinds of real template functions, not just this silly example. It must be a feature of the language, but what?

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 <iostream>
#include <sstream>
#include <string>
using namespace std;

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

    template<typename T>
    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

Answer 2

C++ doesn't do type inference on the return value. I.e., the fact that it is being assigned to an int isn't used in template parameter deduction.

(Removed edit, since someone else presented the overloaded cast solution already.)