std::string can be passed as nlohmann::json to explicit constructor

Question

Why does the following code compile, even though I am passing a std::string object to a explicit constructor which expects a nlohmann::json (to the library) object? My understanding is that std::string won't be implicit converted due to the explicit keyword. Is it possible to change my code so that it will compile successful only when a nlohmann::json is passed?

I use Visual Studio 2019 in Debug Mode with /Wall.

#include <nlohmann/json.hpp>

struct A {
    explicit A(nlohmann::json json) {

    }
};

int main() {
    std::string a = "hello";
    A b(a);
}

Remy Lebeau · Accepted Answer · 2020-01-13T18:53:49.023

Why does the following code compile, even though I am passing a std::string object to a explicit constructor which expects a nlohmann::json (to the library) object? My understanding is that std::string won't be implicit converted due to the explicit keyword.

The explicit in the A constructor just means that the A constructor must be called explicitly (which you are). The compiler is allowed to use implicit conversions when passing arguments to the A constructor, unless they use types that are also explicit themselves (which the nlohmann::json constructor is not).

Is it possible to change my code so that it will compile successful only when a nlohmann::json is passed?

You can pass the argument by a non-const reference, preventing the compiler from passing an implicitly created temporary object:

struct A {
    explicit A(nlohmann::json &json) {
    }
};

So there's no possibility to change this behaviour without altering the source code of `nlohmann::json`? — DasElias, Jan 13 '20 at 18:50

score 0 · Answer 2 · answered Dec 06 '22 at 06:25

This might be overkill but the following appears to work while also allowing a const-reference as constructor argument:

#include <nlohmann/json.hpp>
#include <type_traits>

struct A {
    template<typename T, typename = std::enable_if_t<std::is_same_v<T, nlohmann::json>>>
    explicit A(const T& json) {

    }
};

int main() {
    const std::string a = "hello";
    // A b(a); // ERROR
    
    const auto json_val = nlohmann::json::parse("{}") ;
    A c {json_val} ;
}

std::string can be passed as nlohmann::json to explicit constructor

2 Answers2