SFINAE of a std:array of std::vector doesn't compile in C++11

Question

I use the following methods to write objects in json format:

#include <array>
#include <vector>
#include <jsoncpp/json/json.h>

//// Write json SFINAE
template <typename T>
struct has_write_json_method {
    template <typename U>
    static constexpr decltype(std::declval<U>().write_json(), bool()) test(int) { return true; };

    template <typename U>
    static constexpr bool test(...) { return false; }

    static constexpr bool value = test<T>(int());
};

template <class T>
typename std::enable_if<has_write_json_method<T>::value, Json::Value>::type write_json(const T& object) { return object.write_json(); };

template <class T>
typename std::enable_if<!has_write_json_method<T>::value, Json::Value>::type write_json(const T& object);

//// Write json vector
template <class T>
Json::Value write_json(const std::vector<T>& object_v) {
    Json::Value output;
    for (int i = 0; i < object_v.size(); ++i) { output[i] = write_json<T>(object_v.at(i)); };
    return output;
};

//// Write json array
template <class T, std::size_t N>
Json::Value write_json(const std::array<T, N>& object_v) {
    Json::Value output;
    for (int i = 0; i < object_v.size(); ++i) { output[i] = write_json<T>(object_v.at(i)); };
    return output;
};

//// Write json basic
template <class T>
Json::Value write_json_basic(const T& object) {
    Json::Value output;
    output = object;
    return output;
};

template<>
Json::Value write_json<Json::Value>(const Json::Value& object) { return write_json_basic(object); };

template<>
Json::Value write_json<double>(const double& object) { return write_json_basic(object); };

However, when I try to write a std::array of std::vector:

std::array<std::vector<double>, 4> foo_av;
Json::Value output = write_json(foo_av);

It doesn't compile in C++11:

undefined reference to `std::enable_if<!has_write_json_method<std::vector<double, std::allocator<double> > >::value, Json::Value>::type write_json<std::vector<double, std::allocator<double> > >(std::vector<double, std::allocator<double> > const&)'

Update

A reproducible example using std::string instead of Json::value:

#include <array>
#include <vector>
#include <string>

//// Write string SFINAE
template <typename T>
struct has_write_string_method {
    template <typename U>
    static constexpr decltype(std::declval<U>().write_string(), bool()) test(int) { return true; };

    template <typename U>
    static constexpr bool test(...) { return false; }

    static constexpr bool value = test<T>(int());
};

template <class T>
typename std::enable_if<has_write_string_method<T>::value, std::string>::type write_string(const T& object) { return object.write_string(); };

template <class T>
typename std::enable_if<!has_write_string_method<T>::value, std::string>::type write_string(const T& object);

//// Write string vector
template <class T>
std::string write_string(const std::vector<T>& object_v) {
    std::string output;
    for (int i = 0; i < object_v.size(); ++i) { output += write_string<T>(object_v.at(i)); };
    return output;
};

//// Write string array
template <class T, std::size_t N>
std::string write_string(const std::array<T, N>& object_v) {
    std::string output;
    for (int i = 0; i < object_v.size(); ++i) { output += write_string<T>(object_v.at(i)); };
    return output;
};

//// Write string basic
template <class T>
std::string write_string_basic(const T& object) {
    std::string output;
    output = object;
    return output;
};

template<>
std::string write_string<double>(const double& object) { return write_string_basic(object); };


int main () {
    std::array<std::vector<double>, 4> foo_av;
    std::string output = write_string(foo_av);
    
    return 0;
}

Compiling with Godbolt.org, it fails with x86-64 gcc 10.2:

/opt/compiler-explorer/gcc-10.2.0/bin/../lib/gcc/x86_64-linux-gnu/10.2.0/../../../../x86_64-linux-gnu/bin/ld: /tmp/ccAR3MdL.o: in function `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > write_string<std::vector<double, std::allocator<double> >, 4ul>(std::array<std::vector<double, std::allocator<double> >, 4ul> const&)':
/home/ce/<source>:35: undefined reference to `std::enable_if<!has_write_string_method<std::vector<double, std::allocator<double> > >::value, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::type write_string<std::vector<double, std::allocator<double> > >(std::vector<double, std::allocator<double> > const&)'
collect2: error: ld returned 1 exit status

Answer 1

This is because you send T to the write string method, but the arguments you pass don't match T:

for (int i = 0; i < object_v.size(); ++i) {
    //        here --------v 
    output += write_string<T>(object_v.at(i));
}

Here the T you send is correctly std::vector<double>, but there is no write_string<std::vector<double>>() that takes a std::vector<double>.

What you intended to was the compiler to choose this function:

template <class T>
std::string write_string(const std::vector<T>& object_v) {
    // ...
};

But it does not match. You see, the T here is the vector's template argument. So as a result, write_string<std::vector<T>> receive a std::vector<std::vector<T>>.

What can you do to fix the problem? The solution would be to let the compiler deduce the arguments, and not specialize function and overload instead:

for (int i = 0; i < object_v.size(); ++i) {
    // no template args --v 
    output += write_string(object_v.at(i));
}

Live example