Constructor ill-formness and implicit deletion - c++

Question

I'm trying to define some classes but I get some errors I'm not being able to decode.
I do not understand why it states I'm using Test's constructor when I am not.
test.cpp: In constructor 'Test2::Test2(int)':
test.cpp:12:34: error: use of deleted function 'Test::Test()'
explicit Test2(const int line) {}
^
test.cpp:3:7: note: 'Test::Test()' is implicitly deleted because the default definition would be ill-formed:
class Test : public std::runtime_error {
^
test.cpp:3:7: error: no matching function for call to 'std::runtime_error::runtime_error()'
test.cpp:3:7: note: candidates are:
In file included from test.cpp:1:0:
C:/Development/MinGW/x86_64-w64-mingw32/include/c++/stdexcept:119:5: note: std::runtime_error::runtime_error(const string&)
runtime_error(const string& __arg);
^
C:/Development/MinGW/x86_64-w64-mingw32/include/c++/stdexcept:119:5: note: candidate expects 1 argument, 0 provided
C:/Development/MinGW/x86_64-w64-mingw32/include/c++/stdexcept:112:9: note: std::runtime_error::runtime_error(const std::runtime_error&)
class runtime_error : public exception
^
C:/Development/MinGW/x86_64-w64-mingw32/include/c++/stdexcept:112:9: note: candidate expects 1 argument, 0 provided
Minimal working example (gcc 4.9.1, g++ -std=c++11):
#include <stdexcept>
class Test : public std::runtime_error {
public:
virtual ~Test() noexcept {}
virtual const char * what() const noexcept = 0;
};
class Test2 : public Test {
public:
explicit Test2(const int line) {}
virtual const char * what() const noexcept { return ""; }
};

Answer 1

Your constructor in the class Test2 does not pass any arguments to the base class. So the default constructor of the base class - which is Test - is used. And the same story with Test then - its default constructor tries to call the default constructor of its base class - std::runtime_error - which does not exist. Hence the error. You need to pass message string argument to std::runtime_error, e.g.:
Test() : std::runtime_error("some message") {}