What is the difference between include and declaration?

For example, in the sentence below,

#include<iostream>
# include <cmath>

float Myabs (float x);

int main() {
    float a = 2.5;
    std::cout<<Myabs(a)<<std::abs(a);
    a = -5.25;
    std::cout<<Myabs(a)<<std::abs(a);
    return 0;
}

float Myabs (float x) {return x > 0?x:-x;}

If you change this sentence as shown in the two examples below,
Example 1

#include<iostream>

float Myabs (float x);

int main() {
    float a = 2.5;
    std::cout<<Myabs(a)<<std::abs(a);
    a = -5.25;
    std::cout<<Myabs(a)<<std::abs(a);
    return 0;
}
# include <cmath>
float Myabs (float x) {return x > 0?x:-x;}

Example 2

#include<iostream>
# include <cmath>

int main() {
    float a = 2.5;
    std::cout<<Myabs(a)<<std::abs(a);
    a = -5.25;
    std::cout<<Myabs(a)<<std::abs(a);
    return 0;
}
float Myabs (float x);
float Myabs (float x) {return x > 0?x:-x;}

Both of them have errors in both cases.Error due to lack of declaration and error due to lack of inclusion.
Does this indicate that the include statement and declaration behave the same way from the compiler perspective?

c++ preprocessor

2022-09-30 10:44

2 Answers

Try the -E compilation option.Prints source code processed by the preprocessor.The source code output by this is the source code of the C and C++ languages.You can understand that preprocessor instructions such as #include and #define#if#endif have been processed and disappeared.

2022-09-30 10:44

First, include and declaration are different.
#include only has the ability to capture the contents of a file during compilation.
Typically, #include uses it to capture header files.A header file is usually a file with a declaration; iostream and cmath are also header files, which contain declarations such as std::cout and std::abs.This may be confusing.

Example 1

The std::abs declaration is described in <cmath>.However, std::abs(a) appears before #include<cmath> appears, so no declaration is found and an error occurs.

Example 2

Error because Myabs(a) appears before the float Myabs(float x); declaration.

2022-09-30 10:44

If you have any answers or tips

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