Don't add redundant `==` or `!=` to conditions

Reason

Doing so avoids verbosity and eliminates some opportunities for mistakes. Helps make style consistent and conventional.

Example

By definition, a condition in an if-statement, while-statement, or a for-statement selects between true and false. A numeric value is compared to 0 and a pointer value to nullptr.

// These all mean "if p is not nullptr"
if (p) { ... }            // good
if (p != 0) { ... }       // redundant !=0, bad: don't use 0 for pointers
if (p != nullptr) { ... } // redundant !=nullptr, not recommended

Often, if (p) is read as "if p is valid" which is a direct expression of the programmers intent, whereas if (p != nullptr) would be a long-winded workaround.

Example

This rule is especially useful when a declaration is used as a condition

if (auto pc = dynamic_cast<Circle*>(ps)) { ... } // execute if ps points to a kind of Circle, good

if (auto pc = dynamic_cast<Circle*>(ps); pc != nullptr) { ... } // not recommended

Example

Note that implicit conversions to bool are applied in conditions. For example:

for (string s; cin >> s; ) v.push_back(s);

This invokes istream's operator bool().

Note

Explicit comparison of an integer to 0 is in general not redundant. The reason is that (as opposed to pointers and Booleans) an integer often has more than two reasonable values. Furthermore 0 (zero) is often used to indicate success. Consequently, it is best to be specific about the comparison.

void f(int i)
{
    if (i)            // suspect
    // ...
    if (i == success) // possibly better
    // ...
}

Always remember that an integer can have more than two values.

Example, bad

It has been noted that

if(strcmp(p1, p2)) { ... }   // are the two C-style strings equal? (mistake!)

is a common beginners error. If you use C-style strings, you must know the <cstring> functions well. Being verbose and writing

if(strcmp(p1, p2) != 0) { ... }   // are the two C-style strings equal? (mistake!)

would not in itself save you.

Note

The opposite condition is most easily expressed using a negation:

// These all mean "if p is nullptr"
if (!p) { ... }           // good
if (p == 0) { ... }       // redundant == 0, bad: don't use 0 for pointers
if (p == nullptr) { ... } // redundant == nullptr, not recommended

Enforcement

Easy, just check for redundant use of != and == in conditions.

Arithmetic