C++ 为什么我们在 header 文件中包含所有声明

Question

#include <iostream>

When you do this and it becomes the source passing through the preprocessor, our file will be 5k. The compiler doesn't do all this declaration, right? (There are always some things we don't use after all)

| Is the linker or compiler preventing this?

Answer 1

**so when you include all declaration file in header **

1. improve build times.
2. link against code without having the source for the definitions.
3. avoid marking everything "inline".

compiler only see the declaration as it know definition might be somewhere, then it use all definition at the linking time.

Answer 2

The rule of thumb is this: Header files should contain declarations, source files should contain definitions.

Two types of Declarations:

1. DECLARATIONS: A declaration introduces a name into a scope. Generally speaking, a scope is either an entire.cpp file or anything in code delimited by {}, be it a function, a loop within a function, or even an arbitrarily placed block of {} within a function. A name introduced, is visible within the scope from the point at which it is declared to the end of that scope. A declarations merely tells the compiler how to use something, it does not actually create anything.

    extern int y; // declares y, but does not define it.  y is defined elsewhere,
                  // but the program can now use it since it knows what it is (an integer)

2. PROTOTYPES: A prototype is just another name for a declaration of a function.

    double someFunction( double, int );

referred from: http://www.cplusplus.com/articles/yAqpX9L8/

Also get more information on this site: http://www.cplusplus.com/articles/Gw6AC542/