C++: How does it work alias members class? -

C++: How does it work alias members class? -

in next code:

class prova: public std::map< int,int > { public: prova():a(5){} int a; }; int main() { std::vector<prova> p; prova obj; p.push_back(obj); homecoming 0; }

it works without problem; if add together reference fellow member

class prova: public std::map< int,int > { public: prova():a(5){} int a; int& b=a; }; int main() { std::vector<prova> p; prova obj; p.push_back(obj); homecoming 0; }

i have

warning: non-static info fellow member initializers available -std=c++11 or -std=gnu++11 [enabled default]| /home/andrea/scaricati/prova.cpp||in instantiation of ‘void std::vector<_tp,_alloc>::_m_insert_aux(std::vector<_tp, _alloc>::iterator, const _tp&) [with _tp = prova; _alloc = std::allocator<prova>; std::vector<_tp, _alloc>::iterator = __gnu_cxx::__normal_iterator<prova*, std::vector<prova> >; typename std::_vector_base<_tp, _alloc>::pointer = prova*]’:| /usr/include/c++/4.8/bits/stl_vector.h|913|required ‘void std::vector<_tp, _alloc>::push_back(const value_type&) [with _tp = prova; _alloc = std::allocator<prova>; std::vector<_tp, _alloc>::value_type = prova]’| /home/andrea/scaricati/prova.cpp|19|required here| /home/andrea/scaricati/prova.cpp|4|error: non-static reference fellow member ‘int& prova::b’, can’t utilize default assignment operator| /usr/include/c++/4.8/bits/vector.tcc|335|note: synthesized method ‘prova& prova::operator=(const prova&)’ first required here |||=== build failed: 1 error(s), 4 warning(s) (0 minute(s), 0 second(s)) ===|

no problems adding -std=g++11 suggested warning. if remove vector declaration have warning without error. why c++ old standard can't it? problem references attributes? there scheme same things without using c++11 standard? try

class prova: public std::map< int,int > { public: prova():a(5),b(a){} int a; int &b; };

but errors. why this? thanks!

you warning because compiler smart plenty know syntax of c++11. in c++ such syntax int& b= a called in-class fellow member initializer , in previous standards, done static const variables. c++11 relaxes constraints.

from stroustrup's page:

in c++98, static const members of integral types can initialized in-class, , initializer has constant expression. these restrictions ensure can initialization @ compile-time.

for example:

int var = 7; class x { static const int m1 = 7; // ok const int m2 = 7; // error: not static static int m3 = 7; // error: not const static const int m4 = var; // error: initializer not constant look static const string m5 = "odd"; // error: not integral type // ... };

the basic thought c++11 allow non-static info fellow member initialized declared (in class). constructor can utilize initializer when run-time initialization needed. consider:

class { public: int = 7; };

this equivalent to:

class { public: int a; a() : a(7) {} };

c++ class c++11 reference alias