Depending on your C++ compiler, when you start a project, C++ may require a prototype declaration for functions or not.
You really want prototypes! Here's why:
According to some byzantine aspect of C/C++, if you use a function that isn't defined and you don't have C++ requiring prototypes, then C++ will assume that the function (a) exists and (b) takes arguments that are exactly like what you are passing!
Now if the function doens't exist, you don't need prototypes to help you - you'll get a link error later on saying "we never found this function". Prototypes are better because it doesn't take a full compile to find out.
But where things really get ugly is if the function does exist, but the calling conventions you passed in assume an implicit conversion. For example, imagine if your function goes like this:
void some_happy_func(float n);
Now you call it like this:
Without prototypes, this probably will cause a lot of pain. Why? Well, 2.0 is a double-precision argument! If some_happy_func's prototype exists, the compiler will create an implicit conversion. But if it doesn't, the compiler will assume some_happy_func takes doubles, and generate an incorrect stack (with a double on it). When some_happy_func actually runs, it will interpret the low 32 bits of that double as a float and all hell will break loose.
In my experience, when prototypes reuqirements are off it's really easy to lose track of whether you've included a needed header or not, causing possibly subtle bugs where functions fail sometimes based on the calling code's headers. If the failure causes a hard-to-detect bug it can take a while to unravel this.