try, throw, and catch Statements (C++)

To implement exception handling in C++, you use try, throw, and catch expressions.

First, use a try block to enclose one or more statements that might throw an exception.

A throw expression signals that an exceptional condition—often, an error—has occurred in a try block. You can use an object of any type as the operand of a throw expression. Typically, this object is used to communicate information about the error. In most cases, we recommend that you use the std::exception class or one of the derived classes that are defined in the standard library. If one of those is not appropriate, we recommend that you derive your own exception class from std::exception.

To handle exceptions that may be thrown, implement one or more catch blocks immediately following a try block. Each catch block specifies the type of exception it can handle.

This example shows a try block and its handlers. Assume that GetNetworkResource() acquires data over a network connection and that the two exception types are user-defined classes that derive from std::exception. Notice that the exceptions are caught by const reference in the catch statement. We recommend that you throw exceptions by value and catch them by const reference.

MyData md;
try {
   // Code that could throw an exception
   md = GetNetworkResource();
}
catch (const networkIOException& e) {
   // Code that executes when an exception of type
   // networkIOException is thrown in the try block
   // ...
   // Log error message in the exception object
   cerr << e.what();
}
catch (const myDataFormatException& e) {
   // Code that handles another exception type
   // ...
   cerr << e.what();
}

// The following syntax shows a throw expression
MyData GetNetworkResource()
{
   // ...
   if (IOSuccess == false)
      throw networkIOException("Unable to connect");
   // ...
   if (readError)
      throw myDataFormatException("Format error"); 
   // ...
}

The code after the try clause is the guarded section of code. The throw expression throws—that is, raises—an exception. The code block after the catch clause is the exception handler. This is the handler that catches the exception that's thrown if the types in the throw and catch expressions are compatible. For a list of rules that govern type-matching in catch blocks, see How Catch Blocks are Evaluated (C++). If the catch statement specifies an ellipsis (...) instead of a type, the catch block handles every type of exception. When you compile with the /EHa option, these can include C structured exceptions and system-generated or application-generated asynchronous exceptions such as memory protection, divide-by-zero, and floating-point violations. Because catch blocks are processed in program order to find a matching type, an ellipsis handler must be the last handler for the associated try block. Use catch(...) with caution; do not allow a program to continue unless the catch block knows how to handle the specific exception that is caught. Typically, a catch(...) block is used to log errors and perform special cleanup before program execution is stopped.

A throw expression that has no operand re-throws the exception currently being handled. We recommend this form when re-throwing the exception, because this preserves the original exception’s polymorphic type information. Such an expression should only be used in a catch handler or in a function that's called from a catch handler. The re-thrown exception object is the original exception object, not a copy.

try {
   throw CSomeOtherException();
}
catch(...) {
   // Catch all exceptions – dangerous!!!
   // Respond (perhaps only partially) to the exception, then
   // re-throw to pass the exception to some other handler
   // ...
   throw;
}
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