To invoke or call a function in C++, you need to use the function's name and provide any necessary arguments.

Here's the general syntax for calling a function:

function_name(argument_list);

function_name is the name of the function you want to call, and argument_list is a comma-separated list of arguments that the function takes.

For example, let's say you have a function named greet that takes a string parameter and prints a greeting message to the console:

void greet(std::string name) {
    std::cout << "Hello, " << name << "!" << std::endl;
}

To call this function, you would use:

greet("John");

This would print "Hello, John!" to the console.

If the function returns a value, you can store the return value in a variable:

int add(int a, int b) {
    return a + b;
}

int result = add(3, 4);

This calls the add function with arguments 3 and 4, and stores the return value 7 in the result variable.

Note that the argument types and number of arguments passed to the function must match the function declaration or definition. If they don't, you may get a compilation error.