domiyanyue

// example declaration

// declaration of a class
class MyClass;

// delaration of a function
int my_function (int arg);

// declaration of varibales, also contains definition
int my_var;

domiyanyue

// example definition

// definition of a class
class MyClass {
private:
    int m_size;
public:
    MyClass(int n) : m_size(n){}
    ~MyClass() {}
};

domiyanyue

bool is_even(int n);
bool is_odd(int n);

bool is_even(int n) {
    if (n == 0)
        return true;
    else
        return is_odd(n - 1);
}

domiyanyue

example.cpp: In function ‘bool is_even(int)’:
example.cpp:8:16: error: ‘is_odd’ was not declared in this scope
         return is_odd(n - 1);
                ^~~~~~

domiyanyue

class House;
class Owner;

class House{
public:
    Owner* my_owner;
    House(Owner* owner){
        my_owner = owner;
    }
    ~House(){}

domiyanyue

class_example.cpp:6:5: error: ‘Owner’ does not name a type
     Owner* my_owner;
     ^~~~~
class_example.cpp:8:19: error: ‘Owner’ has not been declared
     void setOwner(Owner* owner){
                   ^~~~~
class_example.cpp: In member function ‘void House::setOwner(int*)’:
class_example.cpp:9:9: error: ‘my_owner’ was not declared in this scope
         my_owner = owner;
         ^~~~~~~~

domiyanyue

int a = 1;
int foo(){
    return a;
}

domiyanyue

int bar() {
    return 1;
}

int foo(){
    return bar();
}

domiyanyue

#include "util.h"
int foo(){
    return bar();
}

domiyanyue

int foo(){
    // can't find bar's definition
    return bar();
}

int bar() {
    return 1;
}