Enforcing function call order in C++

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Introduction

When declaring an interface (as a class for example), there is a need to define the allowed calling order of functions. For example, Init() must be called before any other function, and nothing should be called after Shutdown().

In the spirit of design by contract, I wrote this tiny class + macros to make such rules enforceable in runtime.

See also:

• Debugging and testing made easy. (Part 1)

This code was tested on VC 6 and should work on any C++ compiler.

Background

Design By Contract (DBC) is an issue that many people talk about but I did not see much practical work in the environment I work with (C++). DBC is all about clearly defining what your code should do (This might not be the best definition, but it will do for now).

While working on a complex project I found a bug caused by forgetting to call Init() before the function I needed. Of course, it happened only in certain control flows, to make it more challenging.

The solution: Check in runtime that things happen as you expect them to be. Putting ASSERT()s helps but it is not enough by itself.

Using the code

The code consists of one class and a few macros to make the source readable. For example, the statement function B::foo() should be called after Init() and not after shutdown() is written as:

void B::foo()
{
    Enter("foo");
    Require(Called("Init"));
    Require(!Called("shutdown"));
    // do something here
}

The full list of macros you can use (see also the .h file):

class B
{
// ...
private:
    DBC_Data;  // add this macro to your data declaration section
};