Non trivial struct constructor inside a union in C++ -

in following fragment of code receive compiler error when method data::setvalue(int, int) declared virtual:

struct data{     int ma;     int mb;     virtual void setvalues(int a, int b){         ma = a;         mb = b;     } };  struct threadmessage {     enum type {         data     };      type msg_type;     union content {         data d;         int a;     }content; }; 

the error compiler (g++ (ubuntu/linaro 4.6.3-1ubuntu5) 4.6.3) gives me is:

struct.cpp:19:14: error: member 'data threadmessage::content::d' constructor not allowed in union

struct.cpp:19:14: error: member 'data threadmessage::content::d' copy assignment operator not allowed in union

this set of compiler errors took me off guard. in actual code, had many more attributes , functions. therefore, started looking put bloody operator=() , constructor didn't write them in struct data.

i know problem disappears when make data::setvalues not virtual. why struct data have constructor? when struct have constructor in c++? , why compilation error disappear when make virtual void data::setvalues(int, int) non virtual?

the error messages quoted misleading, no doubt wonder why making function non-virtual fixes problem.

first - structs , classes in c++ have constructors , copy assignment operators. if not create them yourself, created you. (you may delete default versions). in example there automatically generated constructor of data, , automatically generated assignment operator.

now why error messages misleading? because not true. in c++ can have union members constructors or assignment operators. problem starts when member functions not trivial. before c++11 not possible have union member non-trivial constructor. c++11 changed it, still without writing additional functions not possible use such union.

a struct/class virtual functions has non-trivial member functions constructor , assignment operator (because hidden data member needs managed). why when make function non-virtual errors disappear - member functions become trivial , struct may used union member without problems.