C++ Exceptions and handling - c++

Question

I am new to writing exceptions in c++ and I am struggling with an error. I won't get very much into detail 'cause I don't think this is relevant for the problem that I have.
#ifndef _STUDENT_H_
#define _STUDENT_H_
#include <string>
#include <map>
#include <vector>
#include <stdexcept>
#include <iostream>
class NoMarkException: public exception
{
public: NoMarkException():exception(){
cout<< "No marks were found." << endl;
}/*
NoMarkException(const string &name){
cout << "No marks for " << name << " were found."<< endl;
}*/
};
#endif
This is my NoMarkException class
float Student::getMaxMark() const throw(NoMarkException) {
if (marks.empty()) {
throw NoMarkException::NoMarkException();
}
float final = 0;
for (it = marks.begin(); it != marks.end(); it++) {
if ((*it).second > final) {
final = (*it).second;
}
}
return final;
}
And this is my Student.cpp
When I am building the project I get error: cannot call constructor 'NoMarkException::NoMarkException' directly
Any ideas why it is causing the problem?

Answer 1

if (marks.empty()) {
throw NoMarkException();
}

Answer 2

The error you get does not actually have anything to do with the fact you're using an exception class. The problem is that you're trying to instantiate an object of class NoMarkException by explicitly calling the (default) constructor, NoMarkException::NoMarkException(). In C++ you don't call constructors explicitly; rather, when you define a new variable, an appropriate constructor gets called. For example
void foo() {
std::vector<int> v(5);
NoMarkException my_exception();
NoMarkException my_other_exception();
}
or, using equivalent but more appropriate due to recent language changes:
void foo() {
std::vector<int> v { 5 };
NoMarkException my_exception { };
NoMarkException my_other_exception { };
}
In this case (both syntax alternatives), the constructors:
std::vector<int>::vector(std::vector<int>::size_type count)
NoMarkException::NoMarkException()
NoMarkException::NoMarkException()
are called (*).
In your case, you simply need to replace the explicit call you make to NoMarkException::NoMarkException() in the throw with an instantiation of NoMarkException object, i.e. your statement will be:
throw NoMarkException();
(*) - actually, the vector constructor that's called has a few more parameters which take their default values. See here.