Assume that your code will run as part of a multi-threaded program

Reason

It's hard to be certain that concurrency isn't used now or won't be used sometime in the future. Code gets reused. Libraries not using threads might be used from some other part of a program that does use threads. Note that this rule applies most urgently to library code and least urgently to stand-alone applications. However, over time, code fragments can turn up in unexpected places.

Example, bad

double cached_computation(int x)
{
    // bad: these statics cause data races in multi-threaded usage
    static int cached_x = 0.0;
    static double cached_result = COMPUTATION_OF_ZERO;

    if (cached_x != x) {
        cached_x = x;
        cached_result = computation(x);
    }
    return cached_result;
}

Although cached_computation works perfectly in a single-threaded environment, in a multi-threaded environment the two static variables result in data races and thus undefined behavior.

Example, good

struct ComputationCache {
    int cached_x = 0;
    double cached_result = COMPUTATION_OF_ZERO;

    double compute(int x) {
        if (cached_x != x) {
            cached_x = x;
            cached_result = computation(x);
        }
        return cached_result;
    }
};

Here the cache is stored as member data of a ComputationCache object, rather than as shared static state. This refactoring essentially delegates the concern upward to the caller: a single-threaded program might still choose to have one global ComputationCache, while a multi-threaded program might have one ComputationCache instance per thread, or one per "context" for any definition of "context." The refactored function no longer attempts to manage the allocation of cached_x. In that sense, this is an application of the Single Responsibility Principle.

In this specific example, refactoring for thread-safety also improved reusability in single-threaded programs. It's not hard to imagine that a single-threaded program might want two ComputationCache instances for use in different parts of the program, without having them overwrite each other's cached data.

There are several other ways one might add thread-safety to code written for a standard multi-threaded environment (that is, one where the only form of concurrency is std::thread):

• Mark the state variables as thread_local instead of static.
• Implement concurrency control, for example, protecting access to the two static variables with a static std::mutex.
• Refuse to build and/or run in a multi-threaded environment.
• Provide two implementations: one for single-threaded environments and another for multi-threaded environments.

Exception

Code that is never run in a multi-threaded environment.

Be careful: there are many examples where code that was "known" to never run in a multi-threaded program was run as part of a multi-threaded program, often years later. Typically, such programs lead to a painful effort to remove data races. Therefore, code that is never intended to run in a multi-threaded environment should be clearly labeled as such and ideally come with compile or run-time enforcement mechanisms to catch those usage bugs early.