How to inherit from std::runtime_error?

Question

For example:

#include <stdexcept>
class A { };
class err : public A, public std::runtime_error("") { };
int main() {
   err x;
   return 0;
}

With ("") after runtime_error I get:

error: expected '{' before '(' token
error: expected unqualified-id before string constant
error: expected ')' before string constant

else (without ("")) I get

In constructor 'err::err()':
error: no matching function for call to 'std::runtime_error::runtime_error()'

What's going wrong?

(You can test it here: http://www.compileonline.com/compile_cpp_online.php)

Solution 1

This is the correct syntax:

class err : public A, public std::runtime_error

And not:

class err : public A, public std::runtime_error("")

As you are doing above. If you want to pass an empty string to the constructor of std::runtime_error, do it this way:

class err : public A, public std::runtime_error
{
public:
    err() : std::runtime_error("") { }
//        ^^^^^^^^^^^^^^^^^^^^^^^^
};

Here is a live example to show the code compiling.

Solution 2

Would just like to add that alternatively the err class could take a string message and simply forward it to std::runtime_error, or an empty string by default, like so:

#pragma once

#include <stdexcept>

class err : public std::runtime_error
{
public:
    err(const std::string& what = "") : std::runtime_error(what) {}
};

Comments

[mchen]

I tried it on compileonline.com/compile_cpp_online.php, and your suggestion gives me no matching function for call to 'std::runtime_error::runtime_error()'

[Andy Prowl]

@MiloChen: Are you sure you copied everything correctly? I added a link to a live example that shows the code compiles correctly

[mchen]

Oh, I see, it won't compile if I miss out the constructor err() : std::runtime_error("") { }. It's not that I want to pass an empty string - I'm forced to.

[Andy Prowl]

@MiloChen: If this answer solves the problem in your question, please consider marking it as accepted :)

[gg99]

you might want to simplify further and inherit std::runtime_error constructor with: using std::runtime_error::runtime_error;