Diagnostic directives

 

C++

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Freestanding and hosted

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Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++20)
noexcept specifier (C++11)
Exceptions
throw-expression	try-catch block
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
decltype (C++11) auto (C++11) alignas (C++11)	const/volatile constexpr (C++11)
Storage duration specifiers
Initialization
Default initialization Value initialization Zero initialization Copy initialization Direct initialization	Aggregate initialization List initialization (C++11) Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Preprocessor

#if#ifdef#ifndef#else#elif#elifdef#elifndef#endif (C++23)(C++23)

#define#undef#,## operators

#include__has_include (C++17)

#error#warning (C++23)

#pragma_Pragma (C++11)

#line

 

Shows the given error message and renders the program ill-formed, or given warning message without affect the validity of the program (since C++23).

Syntax

#error diagnostic-message	(1)
#warning diagnostic-message	(2)	(since C++23)

Explanation

1) After encountering the #error directive, an implementation displays the message diagnostic-message and renders the program ill-formed (the compilation stops).

2) Same as (1), except that the validity of the program is not affected and the compilation continues.

diagnostic-message can consist of several words not necessarily in quotes.

Notes

Before its standardization in C++23, #warning has been provided by many compilers in all modes as a conforming extension.

Example

Run this code

#if __STDC_HOSTED__ != 1
#  error "Not a hosted implementation"
#endif
 
#if __cplusplus >= 202302L
#  warning "Using #warning as a standard feature"
#endif
 
#include <iostream>
int main()
{
    std::cout << "The implementation used is hosted";
}

Possible output:

The implementation used is hosted

References

• C++20 standard (ISO/IEC 14882:2020):

• 15.8 Error directive [cpp.error]

• C++17 standard (ISO/IEC 14882:2017):

• 19.5 Error directive [cpp.error]

• C++14 standard (ISO/IEC 14882:2014):

• 16.5 Error directive [cpp.error]

• C++11 standard (ISO/IEC 14882:2011):

• 16.5 Error directive [cpp.error]

• C++03 standard (ISO/IEC 14882:2003):

• 16.5 Error directive [cpp.error]

• C++98 standard (ISO/IEC 14882:1998):

• 16.5 Error directive [cpp.error]