In a C++ class and can't figure out whats wrong with my code. Involves functions & if/else if statements

In a C++ class and can't figure out whats wrong with my code. Involves
functions & if/else if statements

The assignment is to write a program that will allow the user to calculate
the area and volume of various shapes. Not allowed to use any global
variables with the one exception for PI.
This is my code:
#include <iostream>
#include <iomanip>
using namespace std;
//Functions
void showMenu(int &);
double area (double, double);
double area (double);
double volume (double, double, double);
double volume (double);
int main()
{
int choice;
double area, volume;
const double PI = 3.14;
do
{
showMenu();
cin >> choice;
if (choice < 1 || choice > 5 )
{
cout << "Please select a valid choice of 1-5: " << endl;
cin >> choice;
}
else if (choice == 1)
{
area = double area (double length, double width);
cout << "The area of the rectangle is: " << endl;
}
else if (choice == 2)
{
area = double area (double radius);
cout << "The area of the circle is: " << endl;
}
else if (choice == 3)
{
volume = double volume (double length, double width, double height);
cout << "The volume for a box is: " << endl;
}
else if (choice == 4)
{
volume = double volume (double radius);
cout << "The volume of a sphere is: " << endl;
}
}
while (choice != 5);
return 0;
}
void ShowMenu(int &choice)
{
cout << "1. Calculate the area of a rectangle";
cout << "2. Calculate the area of a circle";
cout << "3. Calculate the volume for a box";
cout << "4. Calculate the volume of a sphere";
cout << "5. Quit";
}
double area (double length, double width);
{
cout << "Enter the length: ";
cin >> length;
cout << "Enter the width: ";
cin >> width;
area = lenght * width;
}
double area (double radius);
{
cout << "Enter the radius: ";
cin >> radius;
area = PI * (radius * 2);
}
double volume (double length, double width, double height);
{
cout << "Enter the length: ";
cin >> length;
cout << "Enter the width: ";
cin >> width;
cout << "Enter the height: ";
cin >> height;
volume = length * width * height;
}
double volume (double radius);
{
cout << "Enter the radius: ";
cin >> radius;
volume = (4/3) * PI * (radius * 3)
}
The errors im getting.
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(20): error C2660: 'showMenu' :
function does not take 0 arguments
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(31): error C2062: type 'double'
unexpected
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(38): error C2062: type 'double'
unexpected
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(45): error C2062: type 'double'
unexpected
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(52): error C2062: type 'double'
unexpected
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(71): error C2447: '{' : missing
function header (old-style formal list?)
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(80): error C2447: '{' : missing
function header (old-style formal list?)
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(87): error C2447: '{' : missing
function header (old-style formal list?)
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\labfour.cpp(98): error C2447: '{' : missing
function header (old-style formal list?)
1> Lab4.cpp 1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\lab4.cpp(117): warning C4244: '=' : conversion
from 'double' to 'float', possible loss of data
1>c:\users\dylan\documents\visual studio
2010\projects\lab4\lab4\lab4.cpp(127): error C2447: '{' : missing function
header (old-style formal list?)