C++11-related compilation error with g++ 4.6 and boost::unordered_map

Question

I have some piece of C++ code that fails with a compilation error unless it is run in C++11 mode, but I can't figure out why that would be the case as the code does not (explicitly) use C++11 features: ``` #include #include #include struct SomeStruct { boost::unordered_map intMap; }; int main(int argc, const char* argv[]) { std::vector vals; vals.resize(100); } ``` When compiling this on an Ubuntu 12.04 64bit machine with gcc 4.6.3-1ubuntu5 and boost 1.48: ``` g++ test.cpp ``` then I get this compilation error: ``` In file included from /usr/include/c++/4.6/vector:61:0, from test.cpp:1: /usr/include/c++/4.6/bits/stl_algobase.h: In function 'typename __gnu_cxx::__enable_if<(! std::__is_scalar<_Tp>::__value), void>::__type std::__fill_a(_ForwardIterator, _ForwardIterator, const _Tp&) [with _ForwardIterator = SomeStruct*, _Tp = SomeStruct, typename __gnu_cxx::__enable_if<(! std::__is_scalar<_Tp>::__value), void>::__type = voi…

Accepted Answer

The copy-assignment operator of `boost::unordered_map` is declared as:

```
unordered_map& operator=(unordered_map &t)
```

which in turn causes (C++98 12.8 [#10]) the implicit copy-assignment operator of `SomeStruct` to be declared as:

```
SomeStruct &operator= (SomeStruct &);
```

Adding an explicit copy-assignment operator

```
SomeStruct &operator= (const SomeStruct &x) { intMap = x.intMap; return *this; }
```

solves the issue, and it works by invoking a different overload of `unordered_map::operator=`.

This issue is fixed in Boost 1.52. which declares the operator as:

```
unordered_map& operator=(unordered_map const& x);
```

C++11-related compilation error with g++ 4.6 and boost::unordered_map

Solution

Problem