On 1 July 2013 22:42, Andrew Ho wrote:
This allows this to be done:
val1 + val2 = val3;
changing the return type to const T would allow the compiler to catch this, but now doing val1 + val2 + val3 must always use copy semantics instead of move semantics.
I can't think of a way which would fix this issue without breaking the use of move semantics.
You can put an lvalue ref-qualifier on the assignment operator, to forbid assignment to rvalues. T& operator=(const T&)&; I'm not sure it's really beneficial to do so though.
How should we proceed with this? It's obviously bad usage, but the compiler should be able to catch this since it's easy to mistype + as =.
Interestingly enough, doing testing with std::string shows the same "problem":
std::string str1("hello"), str2(" "), str3("world"); str1 + str2 = str3;
I tested this issue with std::string and found it in vs2012, gcc 4.3.2, and mingw/gcc 4.8.0. I don't have access to a compiler which pre-dates C++0x/C++11 so I don't know if this issue has always been present. Compiling with gcc 4.8.0 with - std=c++03 still has the issue.
The example is correct C++03. I'm not aware of it being a big problem, despite being part of the language forever.