implicit-declaration

Recently I've learnt about implicit function declarations in C . The main idea is clear but I have some troubles with understanding of the linkage process in this case. Consider the following code ( file a.c ): #include <stdio.h> int main() { double someValue = f(); printf("%f\n", someValue); return 0; } If I try to compile it: gcc -c a.c -std=c99 I see a warning about implicit declaration of function f() . If I try to compile and link: gcc a.c -std=c99 I have an undefined reference error. So everything is fine. Then I add another file (file b.c ): double f(double x) { return x; } And invoke

I read in the accepted answer here that: [a] copy constructor and copy assignment operator won't be generated for a class that explicitly declares a move constructor or move assignment operator I do notice (g++ 4.7.2) that if you define a move constructor, it will be used with, e.g., push_back() , whereas if all you do is = delete the copy constructor, you don't get an implicit move constructor -- you get an error. [...which leads me to wonder which one (move or copy) is actually used if you don't do anything explicitly...] However, this online reference does not make the same explicit

Does the following program invoke Undefined Behaviour in C? int main() { printf("Printf asking: Where is my declaration ?"); } In the above program there is an implicit declaration of printf() , so is the above code fully standard compliant or it just has some implementation specific behaviour? Yes it does. Not having a declaration in scope is UB. J.2 Undefined behavior — For call to a function without a function prototype in scope where the function is defined with a function prototype, either the prototype ends with an ellipsis or the types of the arguments after promotion are not compatible