Consider the following quote from the C book by Dennis ritchie
All variables must be declared before use, although certain declarations can be made implicitly by content.
It is known that all variables of any type must be declared before using it further. I am unaware with the latter part of the statement that certain declarations can be made implicitly by content.
In C, in general, the variables fall under four basic data types char, int, float, double. How can a variable from these datatypes can be used without any declaration before. Please provide an example that shows implicit declaration based on content the variable holds.