simple c++ input function - c++

Question

i am new to c++ from other languages, and looking at examples this code looked like it should work
#include <iostream>
using namespace std;
main()
{
string input = "";
cout << "in: ";
getline(cin, input);
input_recv(input);
}
input_recv(input)
{
if (input == "hello"){
cout << "derp" << endl;
}
}
it will not let me use the function input_recv. it gives me several errors in my IDE. one being `input_recv' undeclared (first use this function). basically what i am trying to do for this is make it respond to input using a function.
EDIT:
#include <iostream>
#include <string>
using namespace std;
void input_recv(string);
int main()
{
while (1 == 1){
string input = "";
cout << "in: ";
getline(cin, input);
input_recv(input);
cin.get();
}
}
void input_recv(string input){
if (input == "hello"){
cout << "derp" << endl;
}
}
thanks

Answer 1

C++ requires the function to be declared before it's used, so if you move the input_recv definition above the main function, it will work. Otherwise, you can leave the program the way it is and add a forward declaration above main like this:
void input_recv(string);
int main()
{
...
}
void input_recv(string input)
{
...
}
Edit:
There are a few other errors here as well as other comments pointed out. One, functions should have a return type and parameter types specified. Also, before using the string type, you need to
#include <string>.

Answer 2

Declare the function first, and use a correct function prototype, here there is not type for input, no return type ... Example below,
#include <iostream>
#include <string>
void input_recv(const std::string& input);
int main()
{
std::string input = "";
std::cout << "in: ";
std::getline(std::cin, input);
input_recv(input);
return 0;
}
void input_recv(const std::string& input)
{
if (input == "hello"){
cout << "derp" << endl;
}
}

Answer 3

C++ is a strongly-typed language. You must declare your variables and your functions with explicit types:
// forward declare your function
void input_recv(std::string input);
// alternatively
void input_recv_better(const std::string& input);
int main()
{
std::string input;
std::cout << "In: ";
std::getline(std::cin, input);
input_recv(input);
input_recv_better(input);
return 0;
}
void input_recv(std::string input)
{
if (input == "hello")
{
std::cout << "derp" << std::endl;
}
}
void input_recv_better(const std::string& input)
{
if (input == "hello")
{
std::cout << "derp!" << std::endl;
}
}

Answer 4

There's a few things definitely wrong with this snippet, I will correct them all so you can observe the difference:
#include <iostream>
using namespace std;
void input_recv(string input);
int main()
{
string input = "";
cout << "in: ";
getline(cin, input);
input_recv(input);
}
void input_recv(string input)
{
if (input == "hello"){
cout << "derp" << endl;
}
}
I have added return types to your functions, data types to your parameters, and a forward declaration of the input_recv function so that the main function knows it exists.
You will definitely want to pick up a book like C++ Primer (the latest edition revised for the C++11 standard) before learning bad practice by trying to forgo some sort of standard training.

Answer 5

#include <iostream>
using namespace std;
void input_recv(string input)
{
if (input == "hello"){
cout << "derp" << endl;
}
}
int main()
{
string input = "";
cout << "in: ";
getline(cin, input);
input_recv(input);
return 0;
}