precondition and post condition check

Hello everyone,

I am looking for a good sample about how to implement C++ pre-condition and
post condition check, but can not find a good sample code. Do you have any
referred ones? :-)

Since I can not find, I wrote the patterns in two ways, I am not sure which
is correct and if both are not correct, how to implement this pattern?

Sample code 1,

[Code]
#define MAX 1024

class Base
{
public:
    void foo(int i)
    {
    if (i > MAX)
    {
        // error handling
    }
    else
    {
        do_foo(i);
    }
}

private:
    virtual void do_foo(int i) = 0;
};

class Derived : public Base
{
private:
    virtual void do_foo(int i)
    {
    // i is never > MAX here
    }
};

int main()
{
    Derived d;
    d.foo (1000);

    return 0;
}
[/Code]

Sample 2,

[Code]
#define MAX 1024

class Base
{
public:
    void foo(int i)
    {
    if (i > MAX)
    {
        // error handling
    }
    else
    {
        do_foo(i);
    }
}

private:
    virtual void do_foo(int i) = 0;
};

class Derived : public Base
{
public:
    virtual void do_foo(int i)
    {
        foo (i);
        // i is never > MAX here
    }
};

int main()
{
    Derived d;
    d.do_foo (1000);

    return 0;
}
[/Code]

thanks in advance,
George