c++ - What is a formal parameter?

Question

When compiling in C++ I often end up with error messages dealing with "formal parameters", such as

error C2719: 'b': formal parameter with __declspec(align('16')) won't be aligned

I do understand the error, and the fact that b is a parameter of a function I am defining.

However, what does it mean that a parameter is formal? Can there be informal parameters as well?

I do notice that the term "formal parameter" appears in other languages as well, so I presume it is a more generic term not necessarily specific to C-family of languages? Are informal parameters supported by some subset of languages?

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Upon seeing the answers, one final question: Where those names formal parameter and actual parameter origin from? Does it origin from the C standard, or is it an effect of calling it as such in some abstract language calculus?

Answer 1

There are formal and actual parameters:

void foo(int arg); //arg is a formal parameter

int main()
{
    int val = 1;
    foo(val);  //val is an actual parameter
}

From C++ Standard:

1.3.1 formal parameter (parameter)

an object or reference declared as part of a function declaration or definition, or in the catch clause of an exception handler, that acquires a value on entry to the function or handler; an identifier from the comma-separated list bounded by the parentheses immediately following the macro name in a function-like macro definition; or a template-parameter. Parameters are also known as formal arguments or formal parameters.

1.3.10 actual parameter (argument)

an expression in the comma-separated list bounded by the parentheses in a function call expression, a sequence of preprocessing tokens in the comma-separated list bounded by the parentheses in a function-like macro invocation, the operand of throw, or an expression, type-id or template-name in the comma-separated list bounded by the angle brackets in a template instantiation. Also known as an actual argument or actual parameter.