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Tip of the Day
Language: C++
Expertise: Beginner
Dec 1, 1999

Implicit Function Declaration

Question:
I am a Computer Science student, and I am trying to compile a program where I use multiple files and my own header file for the first time. But I'm having some trouble. I get an error that says:
 
driver.cc: In function `int main(...)':
driver.cc:34: warning: implicit declaration of function `int dev(...)'
driver.cc:35: warning: implicit declaration of function `int output(...)'
I thought this meant that the function was not defined anywhere else, but I thought the header file and attached file with the definitions did that.

Do you know what I am doing wrong?

Answer:
It looks like you're calling functions before the compiler has seen their declarations/definitions. In C, this is legal (although deprecated). In C++, however, it's illegal. Because many C++ compilers are also C compilers, you receive a warning rather than an error message.

Make sure the prototypes of the functions dev() and output() appear before they are actually called.

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