| Summary: | basic_string assumes that allocators are default-constructible | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Thomas Neumann <tneumann> |
| Component: | libstdc++ | Assignee: | Jonathan Wakely <redi> |
| Status: | RESOLVED FIXED | ||
| Severity: | normal | ||
| Priority: | P3 | ||
| Version: | 4.8.0 | ||
| Target Milestone: | 5.0 | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | 2014-09-19 00:00:00 | |
| Bug Depends on: | |||
| Bug Blocks: | 24882 | ||
| Attachments: | example | ||
This is hardly the only C++11 allocator requirement missing from std::string, ALL of the new requirements are missing, and unlikely to be implemented until we switch to a non-COW string implementation. C++11 allocators are not required to define any of these:
typedef T* pointer;
typedef const T* const_pointer;
typedef T& reference;
typedef const T& const_reference;
typedef unsigned size_type;
typedef int difference_type;
template <class U> struct rebind { typedef MyAllocator<U> other; };
But if your allocator fails to define any of them then std::string (and some of the other containers) will fail to compile. This is a known issue.
(In reply to comment #1) > This is hardly the only C++11 allocator requirement missing from std::string, > ALL of the new requirements are missing, and unlikely to be implemented until > we switch to a non-COW string implementation. Well, but this would be an allocator requirement that is easy to fix. There are only three places with "_Alloc()" in basic_string.tcc, and only two would have to be fixed as far as I can see it. I am not sure yet why the code even tries to construct the allocator, but if necessary we could use is_default_constructible to figure out of the constructor is available. I could try to produce a patch myself if you prefer. But as of new basic_string is quite unusable with stateful allocators. Note that all such changes should be applied to ext/vstring (which will become basic_string in 4.9) not to basic_string. By now the latter is frozen, modulo serious regressions. Fixed for GCC 5 (when using the new string ABI) |
Created attachment 29531 [details] example The empty-string optimization of basic_string assumes that allocators are default constructible. While this used to be the case in C++98, it is no longer true in C++11, as now allocators are allowed to have state. Consider the attached example program. Compiling with g++ -std=c++11 -c t.cpp produces an error message, even though it should compile fine. The problem is the the "_S_construct" calls "_Alloc()", which does not exist. Note that the C++11 standard does not require default constructors. (Section 17.6.3.5, Table 28). In particular, the SimpleAllocator example from Section 17.6.3.5 would trigger the same bug, too.