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The title says it all. I am curious why is the restrict keyword not part of C++ ? I don't know much about C++, and I'm still not able to find anything online that would give a reason blocking this. Does anyone know what terrible things would happen, if a C++ standard would use this keyword similarly to the way C does? Is it just not needed at all?
More explanation: It is not about using it, perhaps I will not have any benefit from this keyword in my whole life. This question is only about curiosity, since restrict is part of C since C99, that is 15 years.
Read this as well: I'm interested in technical reasons, not opinions like "They just didn't like, it is not cool enough"
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In the C programming language (after 99 standard), a new keyword is introduced known as restrict. restrict keyword is mainly used in pointer declarations as a type qualifier for pointers. It doesn't add any new functionality. It is only a way for programmer to inform about an optimization that compiler can make.
In the C programming language, restrict is a keyword, introduced by the C99 standard, that can be used in pointer declarations. By adding this type qualifier, a programmer hints to the compiler that for the lifetime of the pointer, no other pointer will be used to access the object to which it points.
restrict is a keyword in the C99 version of C language and not in C++. In C, A restrict-qualified pointer (or reference) is basically a promise to the compiler that for the scope of the pointer, the target of the pointer will only be accessed through that pointer (and pointers copied from it).
There are several issues in defining "restrict" in C++, some of them are listed in WG paper N3635: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3635.pdf "Towards restrict-like semantics for C++"
Some possible issues with restrict in C++ are:
- Restrict Class members and indirection with “this pointer”
- Passing of restrict qualifiers into functions, functors, lambdas, and templates
- Escaping of restrict pointer values inside functions
- Overlapping array members, strides
Document also list several C++ compilers with limited "restrict" support for C++.
There is also interesting history note in N3635 about non-inclusion of restrict to C++:
At the time of reviewing C99 feature inclusion in C++ during the Mont Tremblant meeting, restrict was considered but was waiting a paper proposal although none came forward....
Restrict is a C99 feature and was never designed to work in class abstractions and it may have to do with that pointers are not common in C++. ... it was designed for fine-grain aliasing for C, but not well-designed for type-based aliasing in C++
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