EXP52-CPP. Do not rely on side effects in unevaluated operands

Some expressions involve operands that are unevaluated. According to the C++ Standard, [expr], paragraph 8 [ISO/IEC 14882-2014]:

In some contexts, unevaluated operands appear. An unevaluated operand is not evaluated. An unevaluated operand is considered a full-expression. [Note: In an unevaluated operand, a non-static class member may be named (5.1) and naming of objects or functions does not, by itself, require that a definition be provided. — end note]

The following expressions do not evaluate their operands: sizeof(), typeid(), noexcept(), decltype(), and declval().

Because an unevaluated operand in an expression is not evaluated, no side effects from that operand are triggered. Reliance on those side effects will result in unexpected behavior. Do not rely on side effects in unevaluated operands.

Note that unevaluated expression operands are used when the declaration of an object is required but the definition of the object is not. For instance, in the following example, the function f() is overloaded, relying on the unevaluated expression operand to select the desired overload, which is then used to determine the result of the sizeof() expression:

int f(int);
double f(double);
 
size_t size = sizeof(f(0));

Noncompliant Code Example (sizeof)

In this noncompliant code example, the expression a++ is not evaluated:

#include <iostream>
void f() {
  int a = 14;
  int b = sizeof(a++);
  std::cout << a << ", " << b << std::endl;
}

Consequently, the value of a after b has been initialized is 14.

Compliant Solution (sizeof)

In this compliant solution, the variable a is incremented outside of the sizeof operator:

#include <iostream>
void f() {
  int a = 14;
  int b = sizeof(a);
  ++a;
  std::cout << a << ", " << b << std::endl;
}

Noncompliant Code Example (decltype)

In this noncompliant code example, the expression i++ is not evaluated within the decltype specifier:

#include <iostream>

void f() {
  int i = 0;
  decltype(i++) h = 12;
  std::cout << i;
}

Consequently, the value of i remains 0.

Compliant Solution (decltype)

In this compliant solution, i is incremented outside of the decltype specifier so that it is evaluated as desired:

#include <iostream>

void f() {
  int i = 0;
  decltype(i) h = 12;
  ++i;
  std::cout << i;
}

Exceptions

EXP52-CPP-EX1: It is permissible for an expression with side effects to be used as an unevaluated operand in a macro definition or substitution failure is not an error (SFINAE) context. Although these situations rely on the side effects to produce valid code, they typically do not rely on values produced as a result of the side effects.

The following code is an example of compliant code using an unevaluated operand in a macro definition:

void small(int x);
void large(long long x);
 
#define m(x) \
  do {
    if (sizeof(x) == sizeof(int)) { \
      small(x); \
    } else if (sizeof(x) == sizeof(long long)) { \
      large(x); \
    } \
  } while (0)
 
void f() {
  int i = 0;
  m(++i);
}

The expansion of the macro m will result in the expression ++i being used as an unevaluated operand to sizeof(). However, the expectation of the programmer at the expansion loci is that i is preincremented only once.

The following code is an example of compliant code using an unevaluated operand in a SFINAE context to determine whether a type can be postfix incremented:

#include <iostream>
#include <type_traits>
#include <utility>

template <typename T>
class is_incrementable {
  typedef char one[1];
  typedef char two[2];
  static one &is_incrementable_helper(decltype(std::declval<typename std::remove_cv<T>::type&>()++) *p);
  static two &is_incrementable_helper(...);
  
public:
  static const bool value = sizeof(is_incrementable_helper(nullptr)) == sizeof(one);
};

void f() {
  std::cout << std::boolalpha << is_incrementable<int>::value;
}

In an instantiation of is_incrementable, the use of the postfix increment operator generates side effects that are used to determine whether the type is postfix incrementable. However, the value result of these side effects is discarded, so the side effects are used only for SFINAE.

Risk Assessment

If expressions that appear to produce side effects are an unevaluated operand, the results may be different than expected. Depending on how this result is used, it can lead to unintended program behavior.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography