how to define a constructor for implicit type conversion in argument?

Question

Recently I was using functions in <filesystem>, e.g. std::filesystem::exists. These functions accept std::filesystem::path. I noticed passing const char* like "/abc" to function like std::filesystem::exists works, e.g. std::filesystem::exists("/abc")

My question is, it seems like when passing const char* to std::filesystem::exists, we are doing some implicit conversion from const char* to filesystem::path.

Which constructor handles this on filesystem::path class?
How can I write a constructor for this type of conversion? Below code doesn't seem to work, and I'm not sure what's wrong. (I'm a bit new to C++).

class A {
  A(const char*& msg) {
    std::cout << msg << std::endl;
  }
};

void func(const A& p) {
}

int main(int argc, const char * argv[]) {
  func("123"); // No matching function for call to 'func'
  return 0;
}

Gaurav Dhiman · Accepted Answer · 2019-12-30 09:17:10Z

1

First of all, make the constructor Public.

And secondly constructor for A in your code is taking const char*& which is reference to const char*, so const char* won't be implicitically converted to A because const char*& is lvalue reference and const char* is rvalue (basically nameless temporary)

Try this it will work

#include<iostream>
class A {
public:
  A(const char* msg) {
    std::cout << msg << std::endl;
  }
};

void func(const A& p) {
}

int main(int argc, const char * argv[]) {
  func("123"); // No matching function for call to 'func'
  return 0;
}

Which constructor handles this on filesystem::path class

According to cppreference, (5) constructor

template< class Source > path( const Source& source, format fmt = auto_format );

takes care of implicit type conversions from const char* to std::filesystem::path

edited Dec 30, 2019 at 9:17

answered Dec 30, 2019 at 8:58

Gaurav Dhiman

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5 Comments

Jaideep Shekhar Over a year ago

Do answer the first part as well.

Daiwei Over a year ago

Ohh, nice. Why doesn't const char*& work? Is there a term for this kind of constructor that I can search for, e.g. is it a copy constructor?

Jaideep Shekhar Over a year ago

@Daiwei it is a parameterised constructor.

Holt Over a year ago

The end of your answer is incorrect. string_type is std::string or std::wstring, so you need a conversion from const char [4] ("abc") to string_type and then to path for exists, which are two user-defined conversions, and the standard only allow one user-defined conversion in an implicit conversion sequence. The constructor used is the 5th one in the link you posted: template< class Source > path( const Source& source, format fmt = auto_format );

Gaurav Dhiman Over a year ago

@Holt thanks for pointing it out, yeah my mistake, updated the answer

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