Clang error: use of typeid requires -frtti (err_no_typeid_with_fno_rtti)

Text	error: use of typeid requires -frtti (since 7.0) error: cannot use typeid with -fno-rtti (until 6.0)
Type	Error
Category	Semantic Issue
Internal Id	err_no_typeid_with_fno_rtti
Internal Message	use of typeid requires -frtti (since 7.0) cannot use typeid with -fno-rtti (until 6.0)
Regular Expression	(?:error|fatal error)\: use of typeid requires \-frtti
First Commit	2012-05-20 1b7f39de3d58 Error when using typeid() with -fno-rtti. PR 12888.

Description

The error is issued by the Clang compiler when it encounters a use of the typeid operator in C++ code without the Run-Time Type Information (RTTI) feature being enabled. RTTI allows types to be identified at runtime and is used by features like typeid and dynamic casts to determine the type of objects. By default, RTTI is enabled in C++, but it can be disabled to reduce binary size and potentially increase performance for applications that do not need runtime type information. Disabling RTTI is done through the -fno-rtti compiler flag. However, using typeid while RTTI is disabled results in this error because typeid relies on RTTI to function properly. To resolve this error, the developer must either refrain from using typeid in the code or enable RTTI by removing the -fno-rtti flag or explicitly enabling RTTI with the -frtti flag. This diagnostic message was updated in version 7.0 of Clang to clarify the required action by specifying the -frtti flag as the solution. Prior to version 6.0, the message was phrased as "cannot use typeid with -fno-rtti," which also indicated the conflict but did not explicitly mention the -frtti flag. In summary, this error safeguards against unintended uses of typeid that would violate the constraints placed by the -fno-rtti flag. It ensures that developers are aware of the requirements and implications of using RTTI features in C++.

AI Generated

Example

In the following example, an attempt is made to use the typeid operator to obtain the type information of a variable inside the main function. The program includes the header <typeinfo>, which defines the typeid operator. The main function declares an integer variable, a, and then attempts to evaluate its type using typeid(a). However, since the compilation flag -fno-rtti is used, indicating that Run-Time Type Information (RTTI) is disabled, this usage of typeid triggers a compilation error. The compiler outputs an error message indicating that the use of typeid requires RTTI to be enabled, which is achieved by either removing the -fno-rtti flag or adding the -frtti flag to the compilation options.

AI Generated

Flags	-xc++ -fno-rtti	[Try out in Compiler Explorer]
Source	#include <typeinfo> int main() { int a; typeid(a); // Use of typeid without enabling RTTI return 0; }
Compiler Output	<source>:5:3: error: use of typeid requires -frtti

Clang Internals (17.0.6)

Git Commit Message

Error when using typeid() with -fno-rtti. PR 12888.

llvm-svn: 157139

Used in Clang Sources

This section lists all occurrences of the diagnostic within the Clang's codebase. For each occurrence, an auto-extracted snipped from the source code is listed including key elements like control structures, functions, or classes. It should illustrate the conditions under which the diagnostic is activated.

clang/lib/Sema/SemaExprCXX.cpp (line 678)

/// ActOnCXXTypeidOfType - Parse typeid( type-id ) or typeid (expression);
ExprResult Sema::ActOnCXXTypeid(SourceLocation OpLoc, SourceLocation LParenLoc, bool isType, void *TyOrExpr, SourceLocation RParenLoc) {
  // ...
  if (!getLangOpts().RTTI) {
    return ExprError(Diag(OpLoc, diag::err_no_typeid_with_fno_rtti));

Triggered in Clang Tests

This section lists all internal Clang test cases that trigger the diagnostic.

clang/test/SemaCXX/no-rtti.cpp

• clang/test/SemaCXX/no-rtti.cpp:9:9: error: use of typeid requires -frtti