C++ Exception Types

The C++ standard library comes with its own hierarchy of exception classes. These give us a standardized type of object to throw within our exceptions.

All exceptions in the standard library inherit from std::exception, but it is typical to use a more specific subclass. Options include classes like std::logic_error and std::runtime_error.

These have yet more specific subclasses - for example, std::logic_error has a subclass called std::invalid_argument

How we classify our exceptions generally doesn’t matter, but it’s helpful to be consistent.

Throwing and Catching std::exception objects

The standard library exceptions are available after including stdexcept:

1#include <stdexcept>
2

Throwing exception objects works in exactly the same way as throwing any other. These classes have a constructor that we can pass a string to, explaining the issue:

1#include <iostream>
2#include <stdexcept>
3
4int Divide(int x, int y) {
5  if (y == 0) {
6    throw std::invalid_argument {
7      "Cannot divide by zero"
8    };
9  }
10  return x/y;
11}
12

Similarly, we catch them like any other. std::exceptions have a what method that allows us to access the description of the exception:

1int main() {
2  try {
3    Divide(4,0);
4  } catch (std::invalid_argument& e) {
5    std::cout << "Invalid Argument: "
6              << e.what();
7  }
8}
9

1Invalid Argument: Cannot divide by zero
2

Catching By Reference

In the above examples, note that we catch the errors by reference, denoted by the inclusion of the & character:

1catch (std::invalid_argument& e)
2

By convention, we should generally be catching errors by reference.

This is important as, just like functions, catch blocks will, by default, catch by value. This has undesirable effects, particularly when dealing with inheritance. If we throw a derived type like std::invalid_argument but catch it by the value of a base type like std::exception we can lose data.

Below is an example. Note the two blocks of code are almost identical - the only difference is the first block is catching by reference, whilst the second is catching by value:

1try {
2  throw std::invalid_argument {
3    "Useful Error Message"
4  };
5} catch (std::exception& e) {
6  std::cout << e.what() << std::endl;
7}
8
9try {
10  throw std::invalid_argument {
11    "Useful Error Message"
12  };
13} catch (std::exception e) {
14  std::cout << e.what()  << std::endl;
15}
16

1Useful Error Message
2std::exception
3

Because the second code is copying a std::invalid_argument into the simpler std::exception, it has lost all the data that is not part of the std::exception type. This includes the custom error message. This is referred to as slicing, and it is usually a mistake we want to avoid.

When Multiple Catchers Match

Given the nature of inheritance, it is possible that multiple catch statements match the specific type of exception thrown. When this happens, the first valid catch statement is what is activated. Consider this example:

1int main() {
2  try {
3    throw std::logic_error("Oops!");
4  } catch (std::exception& e) {
5    std::cout << "Exception: " << e.what();
6  } catch (std::logic_error& e) {
7    std::cout << "Logic Error: " << e.what();
8  }
9}
10

Here, we are throwing a logic_error exception. However, because logic_error is a subclass of std::exception, what we are throwing is also a std::exception.

As a result, the catch statement on line 4 is used. Our output is:

1Exception: std::exception
2

In this case, the catcher for std::logic_error is a useless piece of code. It could never be reached because, under the rules of inheritance, if an object is a std::logic_error it is also a std::exception, so it will always be caught by the earlier catch statement.

Our tools may warn us as such:

1main.cpp:7:12: warning: exception of type 'std::logic_error'
2will be caught by earlier handler
3

Because of this “first catcher wins” behavior, we should ensure the more specific catchers are first:

1int main() {
2  try {
3    throw std::logic_error("Oops!");
4  } catch (std::logic_error& e) {
5    std::cout << "Logic Error: " << e.what();
6  } catch (std::exception& e) {
7    std::cout << "Exception:" << e.what();
8  }
9}
10

1Logic Error: Oops!
2

Creating Our Own Exception Classes

We are not restricted to just throwing the std::exception types - we can throw and catch any object type. This allows us to add fields to our errors that are specific to the needs of our application:

1#include <iostream>
2
3using std::string;
4
5class AuthenticationError {
6 public:
7  AuthenticationError(string Email, string Password)
8      : Email(Email), Password(Password) {}
9
10  string Message{"A user failed to log in"};
11  string Email;
12  string Password;
13};
14
15int main() {
16  try {
17    throw AuthenticationError {
18      "test@email.com", "something-wrong"
19    };
20  } catch (AuthenticationError& e) {
21    std::cout << e.Message
22              << "\n  E-Mail: " << e.Email
23              << "\n  Password: " << e.Password;
24  }
25}
26

1A user failed to log in
2  E-Mail: test@email.com
3  Password: something-wrong
4

Typically, when creating our own errors, we will want to maintain a hierarchy of errors, similar to std::exception. This allows us to create more generic catch statements, by having them catch the more generic types.

It’s also common for teams just to use the standard library’s implementation for this purpose, and to insert their own custom errors into the std::exception hierarchy.

When doing that, we should adopt the convention of making our error message available through the what() function, in the same way std::exception does:

1class AuthenticationError : public std::exception {
2 public:
3  AuthenticationError(string Email, string Password)
4      : Email(Email), Password(Password) {}
5
6  const char* what() const noexcept override {
7    return "A user failed to log in";
8  };
9
10  string Email;
11  string Password;
12};
13
14int main() {
15  try {
16    throw AuthenticationError{"test@email.com", "wrong"};
17  } catch (AuthenticationError& e) {
18    std::cout << "Caught by a specific handler:\n";
19    std::cout << e.what()
20              << "\n  EMail: " << e.Email
21              << "\n  Password: " << e.Password;
22  }
23
24  try {
25    throw AuthenticationError{"test@email.com", "wrong"};
26  } catch (std::exception& e) {
27    std::cout << "\n\nAlso caught by generic handler:\n";
28    std::cout << e.what();
29  }
30}
31

1Caught by specific handler:
2A user failed to log in
3  EMail: test@email.com
4  Password: wrong
5
6Also caught by generic handler:
7A user failed to log in
8