Don't waste time or space

Reason

This is C++.

Note

Time and space that you spend well to achieve a goal (e.g., speed of development, resource safety, or simplification of testing) is not wasted. "Another benefit of striving for efficiency is that the process forces you to understand the problem in more depth." - Alex Stepanov

Example, bad

struct X {
    char ch;
    int i;
    string s;
    char ch2;

    X& operator=(const X& a);
    X(const X&);
};

X waste(const char* p)
{
    if (!p) throw Nullptr_error{};
    int n = strlen(p);
    auto buf = new char[n];
    if (!buf) throw Allocation_error{};
    for (int i = 0; i < n; ++i) buf[i] = p[i];
    // ... manipulate buffer ...
    X x;
    x.ch = 'a';
    x.s = string(n);    // give x.s space for *p
    for (gsl::index i = 0; i < x.s.size(); ++i) x.s[i] = buf[i];  // copy buf into x.s
    delete[] buf;
    return x;
}

void driver()
{
    X x = waste("Typical argument");
    // ...
}

Yes, this is a caricature, but we have seen every individual mistake in production code, and worse. Note that the layout of X guarantees that at least 6 bytes (and most likely more) are wasted. The spurious definition of copy operations disables move semantics so that the return operation is slow (please note that the Return Value Optimization, RVO, is not guaranteed here). The use of new and delete for buf is redundant; if we really needed a local string, we should use a local string. There are several more performance bugs and gratuitous complication.

Example, bad

void lower(zstring s)
{
    for (int i = 0; i < strlen(s); ++i) s[i] = tolower(s[i]);
}

This is actually an example from production code. We can see that in our condition we have i < strlen(s). This expression will be evaluated on every iteration of the loop, which means that strlen must walk through string every loop to discover its length. While the string contents are changing, it's assumed that tolower will not affect the length of the string, so it's better to cache the length outside the loop and not incur that cost each iteration.

Note

An individual example of waste is rarely significant, and where it is significant, it is typically easily eliminated by an expert. However, waste spread liberally across a code base can easily be significant and experts are not always as available as we would like. The aim of this rule (and the more specific rules that support it) is to eliminate most waste related to the use of C++ before it happens. After that, we can look at waste related to algorithms and requirements, but that is beyond the scope of these guidelines.

Enforcement

Many more specific rules aim at the overall goals of simplicity and elimination of gratuitous waste.

• Flag an unused return value from a user-defined non-defaulted postfix operator++ or operator-- function. Prefer using the prefix form instead. (Note: "User-defined non-defaulted" is intended to reduce noise. Review this enforcement if it's still too noisy in practice.)