According to the C++ Standard, [except.spec] paragraph 8 [ISO/IEC 14882-2014]:

A function is said to allow an exception of type E if the constant-expression in its noexcept-specification evaluates to false or its dynamic-exception-specification contains a type T for which a handler of type T would be a match (15.3) for an exception of type E.

If a function throws an exception other than one allowed by its exception-specification it can lead to an implementation-defined termination of the program ([except.spec] paragraph 9).

If a function declared with a dynamic-exception-specification throws an exception of a type that would not match the exception-specification, the function std::unexpected() is called. The behavior of this function can be overridden, but by default causes an exception of std::bad_exception to be thrown. Unless std::bad_exception is listed in the exception-specification, the function std::terminate() will be called.

Similarly, if a function declared with a noexcept-specification throws an exception of a type that would lead to the noexcept-specification to evaluate to false, the function std::terminate() will be called.

Calling std::terminate() leads to implementation-defined termination of the program. To prevent abnormal termination of the program, any function that declares an exception-specification should restrict itself, as well as any functions it calls, to throwing only allowed exceptions.

Noncompliant Code Example

In this noncompliant code example, the second function claims to throw only exception1, but it may also throw exception2.

#include <exception>
 
class exception1 : public std::exception {};
class exception2 : public std::exception {};

void foo() {
  throw exception2; // OK, since foo() promises nothing wrt exceptions
}

void bar() throw (exception1) {
  foo();    // Bad, since foo() can throw exception2
}

Compliant Solution

This compliant solution catches the exceptions thrown by foo():

void bar() throw (exception1) {
  try {
    foo();
  } catch (exception2 e) {
    // handle error, without re-throwing it.
  }
}

Compliant Solution

This compliant solution declares an exception-specification for bar() which covers all of the exceptions that can be thrown from it:

void bar() throw (exception1, exception2) {
  foo();
}

Noncompliant Code Example

In this noncompliant code example, a function is declared as non-throwing, but it is possible for std::vector::resize() to throw an exception when the requested memory cannot be allocated:

#include <vector>
 
void f(std::vector<int> &v, size_t s) noexcept(true) {
  v.resize(s); // May throw 
}

Compliant Solution

In this compliant solution, the function's noexcept-specification is removed, signifying that the function allows all exceptions:

#include <vector>

void f(std::vector<int> &v, size_t s) {
  v.resize(s); // May throw, but that is OK.
}

Implementation Details

Some vendors provide language extensions for specifying whether a function throws or not. For instance, Microsoft Visual Studio provides __declspec(nothrow)), and Clang supports __attribute__((nothrow)). Currently, the vendors do not document the behavior of specifying a nonthrowing function using these extensions. It is presumed that it is undefined behavior when throwing from a function declared with one of these language extensions.

Risk Assessment

Throwing unexpected exceptions disrupts control flow and can cause premature termination and denial of service.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography