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The core C compiler and libraries for building desktop applications that target x86 and x64 systems are included in the VC 2017 v141 toolset (x86, x64). Notable optional tools include support for MFC and C/CLI development. In the following examples, we will show how to create an MFC app, so this optional component was installed. The correct code is code cpp#include /code, try that (and watch the space between them, too). In C you don't add the '.h' for things in the standard library - When can you omit the file extension in an #include directive? In C cppcompiler Fixed In: Visual Studio 2019 version 16.8 fixed in: visual studio 2019 version 16.8 ga fixed in: visual studio 2019 version 16.9 preview 2 fixed in: visual studio 2019 version 16.8 preview 4 visual studio 2019 version 16.8 preview 3 Fixed - Pending Release.

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An exception is a problem that arises during the execution of a program. A C++ exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero.

Exceptions provide a way to transfer control from one part of a program to another. C++ exception handling is built upon three keywords: try, catch, and throw.

throw − A program throws an exception when a problem shows up. This is done using a throw keyword.
catch − A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The catch keyword indicates the catching of an exception.
try − A try block identifies a block of code for which particular exceptions will be activated. It's followed by one or more catch blocks.

Assuming a block will raise an exception, a method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch as follows −

You can list down multiple catch statements to catch different type of exceptions in case your try block raises more than one exception in different situations.

Throwing Exceptions

Exceptions can be thrown anywhere within a code block using throw statement. The operand of the throw statement determines a type for the exception and can be any expression and the type of the result of the expression determines the type of exception thrown.

Following is an example of throwing an exception when dividing by zero condition occurs −

Catching Exceptions

The catch block following the try block catches any exception. You can specify what type of exception you want to catch and this is determined by the exception declaration that appears in parentheses following the keyword catch.

Above code will catch an exception of ExceptionName type. If you want to specify that a catch block should handle any type of exception that is thrown in a try block, you must put an ellipsis, ..., between the parentheses enclosing the exception declaration as follows −

The following is an example, which throws a division by zero exception and we catch it in catch block.

Because we are raising an exception of type const char*, so while catching this exception, we have to use const char* in catch block. If we compile and run above code, this would produce the following result −

C++ Standard Exceptions

C++ provides a list of standard exceptions defined in <exception> which we can use in our programs. These are arranged in a parent-child class hierarchy shown below −

Here is the small description of each exception mentioned in the above hierarchy −

Sr.No	Exception & Description
1	std::exception An exception and parent class of all the standard C++ exceptions.
2	std::bad_alloc This can be thrown by new.
3	std::bad_cast This can be thrown by dynamic_cast.
4	std::bad_exception This is useful device to handle unexpected exceptions in a C++ program.
5	std::bad_typeid This can be thrown by typeid.
6	std::logic_error An exception that theoretically can be detected by reading the code.
7	std::domain_error This is an exception thrown when a mathematically invalid domain is used.
8	std::invalid_argument This is thrown due to invalid arguments.
9	std::length_error This is thrown when a too big std::string is created.
10	std::out_of_range This can be thrown by the 'at' method, for example a std::vector and std::bitset<>::operator[]().
11	std::runtime_error An exception that theoretically cannot be detected by reading the code.
12	std::overflow_error This is thrown if a mathematical overflow occurs.
13	std::range_error This is occurred when you try to store a value which is out of range.
14	std::underflow_error This is thrown if a mathematical underflow occurs.

Define New Exceptions

You can define your own exceptions by inheriting and overriding exception class functionality. Following is the example, which shows how you can use std::exception class to implement your own exception in standard way −

This would produce the following result −

Here, what() is a public method provided by exception class and it has been overridden by all the child exception classes. This returns the cause of an exception.

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In modern C++, in most scenarios, the preferred way to report and handle both logic errors and runtime errors is to use exceptions. It's especially true when the stack might contain several function calls between the function that detects the error, and the function that has the context to handle the error. Exceptions provide a formal, well-defined way for code that detects errors to pass the information up the call stack.

Use exceptions for exceptional code

Program errors are often divided into two categories: Logic errors that are caused by programming mistakes, for example, an 'index out of range' error. And, runtime errors that are beyond the control of programmer, for example, a 'network service unavailable' error. In C-style programming and in COM, error reporting is managed either by returning a value that represents an error code or a status code for a particular function, or by setting a global variable that the caller may optionally retrieve after every function call to see whether errors were reported. For example, COM programming uses the HRESULT return value to communicate errors to the caller. And the Win32 API has the GetLastError function to retrieve the last error that was reported by the call stack. In both of these cases, it's up to the caller to recognize the code and respond to it appropriately. If the caller doesn't explicitly handle the error code, the program might crash without warning. Or, it might continue to execute using bad data and produce incorrect results.

Exceptions are preferred in modern C++ for the following reasons:

An exception forces calling code to recognize an error condition and handle it. Unhandled exceptions stop program execution.
An exception jumps to the point in the call stack that can handle the error. Intermediate functions can let the exception propagate. They don't have to coordinate with other layers.
The exception stack-unwinding mechanism destroys all objects in scope after an exception is thrown, according to well-defined rules.
An exception enables a clean separation between the code that detects the error and the code that handles the error.

The following simplified example shows the necessary syntax for throwing and catching exceptions in C++.

Dev C 2b 2b Application Error Example

Exceptions in C++ resemble ones in languages such as C# and Java. In the try block, if an exception is thrown it will be caught by the first associated catch block whose type matches that of the exception. In other words, execution jumps from the throw statement to the catch statement. If no usable catch block is found, std::terminate is invoked and the program exits. In C++, any type may be thrown; however, we recommend that you throw a type that derives directly or indirectly from std::exception. In the previous example, the exception type, invalid_argument, is defined in the standard library in the <stdexcept> header file. C++ doesn't provide or require a finally block to make sure all resources are released if an exception is thrown. The resource acquisition is initialization (RAII) idiom, which uses smart pointers, provides the required functionality for resource cleanup. For more information, see How to: Design for exception safety. For information about the C++ stack-unwinding mechanism, see Exceptions and stack unwinding.

Basic guidelines

Robust error handling is challenging in any programming language. Although exceptions provide several features that support good error handling, they can't do all the work for you. To realize the benefits of the exception mechanism, keep exceptions in mind as you design your code.

Use asserts to check for errors that should never occur. Use exceptions to check for errors that might occur, for example, errors in input validation on parameters of public functions. For more information, see the Exceptions versus assertions section.
Use exceptions when the code that handles the error is separated from the code that detects the error by one or more intervening function calls. Consider whether to use error codes instead in performance-critical loops, when code that handles the error is tightly coupled to the code that detects it.
For every function that might throw or propagate an exception, provide one of the three exception guarantees: the strong guarantee, the basic guarantee, or the nothrow (noexcept) guarantee. For more information, see How to: Design for exception safety.
Throw exceptions by value, catch them by reference. Don’t catch what you can't handle.
Don't use exception specifications, which are deprecated in C++11. For more information, see the Exception specifications and noexcept section.
Use standard library exception types when they apply. Derive custom exception types from the exception Class hierarchy.
Don't allow exceptions to escape from destructors or memory-deallocation functions.

Exceptions and performance

The exception mechanism has a minimal performance cost if no exception is thrown. If an exception is thrown, the cost of the stack traversal and unwinding is roughly comparable to the cost of a function call. Additional data structures are required to track the call stack after a try block is entered, and additional instructions are required to unwind the stack if an exception is thrown. However, in most scenarios, the cost in performance and memory footprint isn't significant. The adverse effect of exceptions on performance is likely to be significant only on memory-constrained systems. Or, in performance-critical loops, where an error is likely to occur regularly and there's tight coupling between the code to handle it and the code that reports it. In any case, it's impossible to know the actual cost of exceptions without profiling and measuring. Even in those rare cases when the cost is significant, you can weigh it against the increased correctness, easier maintainability, and other advantages that are provided by a well-designed exception policy.

Exceptions versus assertions

Exceptions and asserts are two distinct mechanisms for detecting run-time errors in a program. Use assert statements to test for conditions during development that should never be true if all your code is correct. There's no point in handling such an error by using an exception, because the error indicates that something in the code has to be fixed. It doesn't represent a condition that the program has to recover from at run time. An assert stops execution at the statement so that you can inspect the program state in the debugger. An exception continues execution from the first appropriate catch handler. Use exceptions to check error conditions that might occur at run time even if your code is correct, for example, 'file not found' or 'out of memory.' Exceptions can handle these conditions, even if the recovery just outputs a message to a log and ends the program. Always check arguments to public functions by using exceptions. Even if your function is error-free, you might not have complete control over arguments that a user might pass to it.

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C++ exceptions versus Windows SEH exceptions

Both C and C++ programs can use the structured exception handling (SEH) mechanism in the Windows operating system. The concepts in SEH resemble the ones in C++ exceptions, except that SEH uses the __try, __except, and __finally constructs instead of try and catch. In the Microsoft C++ compiler (MSVC), C++ exceptions are implemented for SEH. However, when you write C++ code, use the C++ exception syntax.

For more information about SEH, see Structured Exception Handling (C/C++).

Exception specifications and `noexcept`

Exception specifications were introduced in C++ as a way to specify the exceptions that a function might throw. However, exception specifications proved problematic in practice, and are deprecated in the C++11 draft standard. We recommend that you don't use throw exception specifications except for throw(), which indicates that the function allows no exceptions to escape. If you must use exception specifications of the deprecated form throw( type-name ), MSVC support is limited. For more information, see Exception Specifications (throw). The noexcept specifier is introduced in C++11 as the preferred alternative to throw().