c++ - How to: copying an instance of a class without a proper default constructor; that has members without a default constructor? -

// b doesn't have default constructor. class {   public:   a(important resource) : b(resource) {}   b b; }; 

reading rule of three figure out issue copying instances of class vector.

so issue we're generating instances inside loop:

std::vector<a> some_vector(0); (int = 0; < 7; i++)   some_vector.push_back(a(resources[i])); 

from see valgrind, smells copy constructor mucking things up. when change code to:

std::vector<a*> some_vector(0); (int = 0; < 7; i++)   some_vector.push_back(new a(resources[i])); 

the issue alleviated. don't doing though when not needed. i'm thinking, how copy constructor know when attempting copy class without proper default constructor right?

for elements of type t pushed std::vector in c++,

t must meet requirements of copyassignable , copyconstructible.

this means must have working move or copy-constructor used when creating new objects. not need default constructor in general, operations may require it, e.g. resize.

being resource in example, should non-copyable, maybe movable. if b not have correct copy-constructor, end copy of resource, , destructor release resource more once (e.g. deallocate memory or close file).

for resources, common pattern use raii, e.g. std::unique_pointer memory.

ps: since c++11, there new type of constructor/assignment operator. rule of 3 becomes rule of five, best not write copy- or assignment constructors @ when not dealing resource-handling classes. these need implement operations, , other classes can use it. rule of zero, , recommend stick to. again, std::unique_pointer example. instead of writing move/copy-constructors, classes can use unique_ptr or shared_ptr, , memory management taken care of.