Error: Undeclared Identifier

Example error message:

doy.cpp: In function 'int main()': doy.cpp:25: 'DayOfYear' undeclared
(first use this function) doy.cpp:25: (Each undeclared identifier is reported only
once for each function it appears in.) doy.cpp:25: parse error before ';' token

Meaning:

You used DayOfYear in your code, but the compiler has not seen a definition for DayOfYear. It doesn't know what DayOfYear is.

Usual Causes

1. You forgot to include the header file that defines the class/struct/function/etc.
2. You misspelled the name of the identifier.

Error: cout Undeclared

Example error message:

xyz.cpp: In function 'int main()': xyz.cpp:6: 'cout' undeclared (first use
this function) xyz.cpp:6: (Each undeclared identifier is reported only once for each
function it appears in.)

Meaning

This is really a special case of the "Undeclared Identifier" error.

Usual Causes

1. You forgot to include <ostream>.
2. You forgot using namespace std;.