在类内声明的C ++ Struct

Question

Long time reader first time poster. Searching for this is very obscure so I couldn't find anything.

Anywho, if I declare a struct inside of a class. Then I have a class method that creates instances of said struct, what is the lifespan of the created struct instance?

For instance in my header:

class test_class
{
     Public:
          void test_function(int, string);

     private:
          struct test_struct
          {
               int foo;
               string bar;
          };

          test_struct * storage;
}

Then in test_class.cpp

void test_function(int num, string name)
{
     test_struct t1;
     t1.foo = num;
     t1.bar = name;

     storage = new test_struct [<some_size>];
     storage[<some_element>] = t1;
}

So, I make an instance of my test_class and call the test_function. t1 is created and then stored successfully in the hypothetical array, but does it stay saved in the array? Or does it get deleted when the function exits because the scope shifts? Does t1 become a member variable of the class? Would I need t1 to be a pointer to a test_struct and have an array of test_struct pointers?

Thank you in advance for the help.

Answer 1

Lifetime has nothing to do with nesting of class definition.

This statement

 test_struct t1;

declares a local variable whose lifetime ends at the closing right brace of the block containing the declaration. It declares a local variable because it's in a block in a function definition.

This statement

 storage = new test_struct [<some_size>];

dynamically creates <some_size> instances of test_struct . The lifetime of that array ends when you delete[] it. If you do.

This statement

storage[<some_element>] = t1;

is a copy assignment, copying the value of t1 . It doesn't create a new instance.

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Answer 2

Looking at your question, looks like you are confused about the struct type inside the class. If it helps, here's a bit of explanation with

      struct test_struct
      {
           int foo;
           string bar;
      };

inside the class, you've just created a new type that's encapsulated inside the class. (Treat it like any other type). Answers to your questions 1.t1 is created and then stored successfully in the hypothetical array, but does it stay saved in the array? Or does it get deleted when the function exits because the scope shifts?

Answer: Yes it copies into the array, as mentioned in the previous answer. Its a copy assignment (bit-wise copy semantics), so, your array's element now has the value copied to it from t1

2. Does t1 become a member variable of the class? Would I need t1 to be a pointer to a test_struct and have an array of test_struct pointers?

Answer: t1 is a local variable (allocated on the stack), its not a member variable of the class. With storage allocated on heap (new) you have an array of elements of test_struct, and then you simply copy the local t1's value to your array. Essentially, you are copying the value of t1 to array before t1 is destroyed when it goes out of scope. You don't need to create a array to pointer to test_struct, (ofcourse if there are some compulsive reasons, in such a case you allocate memory to test_struct as well).

your "storage" member is allocated on heap, so, make sure that you call delete[] to reclaim the memory.

Hope it helps!