Differences between statements and definitions in C ++

Questions from csdn:

I always thought that definition = Declaration + initialization, that is, int I; Is declaration, int I = 0; Is definition.
But according to Hou Jie's TranslationValid C ++The string PS is also a definition (in the section where the clause definition should be as backward as possible). if the definition is too early, the constructor burden will be introduced too early.
Have I ever been wrong with the concepts of declarations and definitions?
If the Declaration does not cause memory allocation, how can we differentiate definitions and declarations?

I have selected several reliable answers in my reply downstairs to summarize them, in case I forget them in the future.

The rigorous C ++ semantics of the statement is used to tell the compiler type and its details, for example:

 
ClassMyclass
 

 
{
 

 
// Data member details...
 

 
// Member function details...
 

 
};
 

The above statement only tells the compiler that there is a custom type of myclass. The Compiler only performs vocabulary analysis and name resolution on it, and does not occupy the memory!

The strict C ++ semantics of "Definition", that is, memory occupation. The compiler will address the object in the relative memory address!

Note that we cannot simply say

 
StringMystring;
 

Whether it is a declaration or definition. The determination principle is to check whether the memory is occupied. For example:

 
ClassMyclass// Class declaration, no memory occupied{StringMystring;// String Declaration};
 

However

 
# Include <iostream>
 

 
// Global scope
 

 
StringMystring;// Definition. mystring is the instantiated string!
 

IntMain ()
 

 
{
 

 
// Main function body
 

 
StringMyanotherstring;// Definition. myanotherstring is the instantiated string!
 

 
Return0;
 

 
}
 

Therefore, I replied:

Variables and objects without extern are always defined, except in classes.

Only function headers are declarations, and function bodies are definitions.

Class is always a declaration. The function body of a class member function is defined.

 
ClassMyclass
 

 
{
 

Static IntX;// Here, X is the Declaration
 

 
Static Const IntA;// Here a is the statement
 

 
// Non-static variables are allocated during class instantiation.
 

 
Myclass ();// The function here is the Declaration
 

 
};
 

 
IntMyclass: X;// This is the definition
 

 
Const IntMyclass: A = 11;// This is the definition
 

How is the distinction made? Hope you can answer your questions.

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