具有原始类型的模板向量类型

Question

I want to template a function with both primitive type ( int , float , double ) and vector type ( vector<int> , vector<float> , vector<double> ). Below is my code. I would like to know if there is a way to template parseKeyValue() with less code duplication when building different vector cases. Thanks!

#include <iostream>
#include <typeinfo>
#include <vector>
#include <string>
#include <sstream>
#include <iterator>


using namespace std;


template<class T> T parseKeyValue(stringstream& ss){
    T value;
    while(ss >> value){};
    return value;
}

template<> vector<string> parseKeyValue(stringstream& ss){
    vector<string> value;
    string item;
    while(ss >> item) value.push_back(item);
    return value;
}

template<> vector<int> parseKeyValue(stringstream& ss){
    vector<int> value;
    int item;
    while(ss >> item) value.push_back(item);
    return value;
}


template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v){
    std::copy(v.begin(), v.end(), std::ostream_iterator<T>(os, " "));
    return os;
}


int main(){

    stringstream ss("1-2-3 7-8-9");
    vector<string> t = parseKeyValue< vector<string> >(ss);
    cout << t << endl;

    stringstream ss2("123 789");
    vector<int> t2 = parseKeyValue< vector<int> >(ss2);
    cout << t2 << endl;

    stringstream ss3("123 789");
    int t3 = parseKeyValue< int >(ss3);
    cout << t3 << endl;

    return 0;
}

Answer 1

You can wrap it in a class template, which could be partial specialized.

template<class T> 
struct Wrapper {
    static T parseKeyValue(stringstream& ss){
        T value;
        while(ss >> value){};
        return value;
    }
};

template<class T> 
struct Wrapper<std::vector<T>> {
    static vector<T> parseKeyValue(stringstream& ss){
        vector<T> value;
        T item;
        while(ss >> item) value.push_back(item);
        return value;
    }
};

template<class T> T parseKeyValue(stringstream& ss){
    return Wrapper<T>::parseKeyValue(ss);
}

LIVE

Or apply SFINAE with template overloading.

template <typename T>
struct is_vector : std::false_type {};
template <typename T>
struct is_vector<std::vector<T>> : std::true_type {};

template<class T> 
std::enable_if_t<!is_vector<T>::value, T>
parseKeyValue(stringstream& ss) {
    T value;
    while(ss >> value){};
    return value;
}

template<class T> 
std::enable_if_t<is_vector<T>::value, T>
parseKeyValue(stringstream& ss) {
    T value;
    typename T::value_type item;
    while(ss >> item) value.push_back(item);
    return value;
}

LIVE

Answer 2

The way you have defined your functions you would need partial specialisations for your functions – which is not legal in C++. Even if it was legal you have no parameters for deducing the template type, so you always would have to specify the template arguments explicitly:

std::vector<int> v = parseKeyValue<std::vector<int>>(...);
//                                ^        ^       ^

as deduction from assignment to variable is not possible.

If you change the function signature to filling a function parameter, you can operate with overloads:

template <typename T>
void parseKeyValue(T& t, std::istream& s);

template <typename T>
void parseKeyValue(std::vector<T>&, std::istream& s);

And you can even have a variant for generic containers:

template <typename T, template <typename> class Container >
void parseKeyValue(Container<T>, std::istream& s);

Note that I changed parameter from std::stringstream to std::istream , which is less restrictive, so you could use the functions with eg std::cin , too.

This way, you do not rely on partial specialisations and additionaly can profit from template argument deduction:

int n;
parseKeyValue(n, std::cin); // selects non-container overload
std::vector<int> v;
parseKeyValue(v, std::cin); // selects vector overload, as more specialised
std::list<int> l;
parseKeyValue(l, std::cin); // selects generic container overload

Side note:

while(ss >> value){};

in your original non-vector version would read any values available and discard all but very last. Is this intended?