c++ - None of the constructors are called on passing Anonymous Object as argument - 


#include <iostream>  struct box {     box()            { std::cout << "constructor called" << std::endl; }     box(const box&)  { std::cout << "copy constructor called" << std::endl; }     box(box&&)       { std::cout << "move constructor called" << std::endl; }     void run() const { std::cout << "run" << std::endl;} };  int main() {     box a(box());     a.run(); }

(demo)

in above code expecting either copy constuctor or move constructor called on passing anonymous object box() argument. none of them called. reason might copy elision. constructor not called anonymous object a(). actually, above code not compile , on calling run() function compiler gave following error.

a.cpp: in function ‘int main()’: a.cpp:28:7: error: request member ‘run’ in ‘a’, of non-class type ‘box(box (*)())’      a.run();

so when type box a(box()) happening? being created?

this case of most vexing parse. when parsed function declaration, is.

box a(box())

is declaration of function named a taking function of type box (*)() argument , returning box.

a solution use (new in c++11) aggregate initialization constructing objects:

box a{box{}}

(demo)

the mvp discussed in simplest form in stackoverflow question most vexing parse: why doesn't a(()); work?

if have expression within valid. example:

((0));//compiles

to learn more how languages defined, , how compilers work, should learn formal language theory or more context free grammars (cfg) , related material finite state machines. if interested in though wikipedia pages won't enough, you'll have book.


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