c++

[harshada]

what is mean by try and catch in c++?

[prem waghmode]

try {
// code that could throw an exception
}
[ catch (exception-declaration) {
// code that executes when exception-declaration is thrown
// in the try block
}
[catch (exception-declaration) {
// code that handles another exception type
} ] . . . ]
// The following syntax shows a throw expression:
throw [expression]

Remarks

The C++ language provides built-in support for handling anomalous situations, known as exceptions, which may occur during the execution of your program. The try, throw, and catch statements implement exception handling. With C++ exception handling, your program can communicate unexpected events to a higher execution context that is better able to recover from such abnormal events. These exceptions are handled by code which is outside the normal flow of control. The Microsoft C++ compiler implements the C++ exception handling model based on the ANSI C++ standard.

C++ also provides a way to explicitly specify whether or a function could throw exceptions. Exception specifications are used in function declarations to indicate that a function could throw an exception. For example, an exception specification throw(...) tells the compiler that a function could throw an exception, but doesn't specify the type. For example:

void MyFunc() throw(...) {
throw 1;
}

[prem waghmode]

Control reaches the try statement by normal sequential execution. The guarded section within the try block is executed.

If no exception is thrown during execution of the guarded section, the catch clauses that follow the try block are not executed. Execution continues at the statement after the last catch clause following the try block in which the exception was thrown.

If an exception is thrown during execution of the guarded section or in any routine the guarded section calls (either directly or indirectly), an exception object is created from the object created by the throw operand. (This implies that a copy constructor may be involved.) At this point, the compiler looks for a catch clause in a higher execution context that can handle an exception of the type thrown (or a catch handler that can handle any type of exception). The catch handlers are examined in order of their appearance following the try block. If no appropriate handler is found, the next dynamically enclosing try block is examined. This process continues until the outermost enclosing try block is examined.

If a matching handler is still not found, or if an exception occurs while unwinding, but before the handler gets control, the predefined run-time function terminate is called. If an exception occurs after throwing the exception, but before the unwind begins, terminate is called.

If a matching catch handler is found, and it catches by value, its formal parameter is initialized by copying the exception object. If it catches by reference, the parameter is initialized to refer to the exception object. After the formal parameter is initialized, the process of unwinding the stack begins. This involves the destruction of all automatic objects that were constructed (but not yet destructed) between the beginning of the try block associated with the catch handler and the exception's throw site. Destruction occurs in reverse order of construction. The catch handler is executed and the program resumes execution following the last handler (that is, the first statement or construct which is not a catch handler). Control can only enter a catch handler through a thrown exception, never via a goto statement or a case label in a switch statement.

The following is a simple example of a try block and its associated catch handler. This example detects failure of a memory allocation operation using the new operator. If new is successful, the catch handler is never executed:

// exceptions_trycatchandthrowstatements.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;
int main() {
char *buf;
try {
buf = new char[512];
if( buf == 0 )
throw "Memory allocation failure!";
}
catch( char * str ) {
cout << "Exception raised: " << str << '\n';
}
}