DCL34-CPP. Do not write syntactically ambiguous declarations

It is possible to devise syntax which can ambiguously be interpreted as either an expression statement or a declaration. Syntax of this sort is referred to as a vexing parse because the compiler must use disambiguation rules to determine the semantic results. The C++ Standard, [stmt.ambig], paragraph 1, states in part [ISO/IEC 14882-2014]:

There is an ambiguity in the grammar involving expression-statements and declarations: An expression-statement with a function-style explicit type conversion as its leftmost subexpression can be indistinguishable from a declaration where the first declarator starts with a (. In those cases the statement is a declaration. [Note: To disambiguate, the whole statement might have to be examined to determine if it is an expression-statement or a declaration. ...

A similarly vexing parse exists within the context of a declaration where syntax can be ambiguously interpreted as either a function declaration, or a declaration with a function-style cast as the initializer. The C++ Standard, [dcl.ambig.res], paragraph 1, states in part:

The ambiguity arising from the similarity between a function-style cast and a declaration mentioned in 6.8 can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8, the resolution is to consider any construct that could possibly be a declaration a declaration.

Do not write a syntactically ambiguous declaration, including vexing parses. With the advent of uniform initialization syntax using a braced-init-list, there is now syntax that unambiguously specifies a declaration instead of an expression statement. Declarations can also be disambiguated by using nonfunction-style casts, initialization using =, or by removing extraneous parenthesis around the parameter name.

Noncompliant Code Example

In this noncompliant code example, an attempt is made to declare a local variable, w, of type Widget while executing the default constructor. However, this is syntactically ambiguous where the code could either be a declaration of a function pointer accepting no arguments and returning a Widget, or a declaration of a local variable of type Widget. The syntax used in this example defines the former instead of the latter.

#include <iostream>
 
struct Widget {
  Widget() { std::cout << "Constructed" << std::endl; }
};

void f() {
  Widget w();
}

As a result, this program compiles and prints no output, because the default constructor is never actually invoked.

Compliant Solution

This compliant solution shows two equally-compliant ways to write the declaration. The first way is to elide the parenthesis after the variable declaration; this ensures the syntax is that of a variable declaration instead of a function declaration. The second way is to use a braced-init-list to direct-initialize the local variable.

#include <iostream>
 
struct Widget {
  Widget() { std::cout << "Constructed" << std::endl; }
};

void f() {
  Widget w1; // Elide the parenthesis
  Widget w2{}; // Use direct initialization
}

Running this program produces the output: Constructed.

Noncompliant Code Example

This noncompliant code example demonstrates a vexing parse. The declaration Gadget g(Widget(i)); is not parsed as declaring a Gadget object with a single argument, but is instead parsed as a function declaration with a redundant set of parenthesis around a parameter. 

#include <iostream>

struct Widget {
  explicit Widget(int I) { std::cout << "Widget constructed" << std::endl; }
};

struct Gadget {
  explicit Gadget(Widget wid) { std::cout << "Gadget constructed" << std::endl; }
};

void f() {
  int i = 3;
  Gadget g(Widget(i));
  std::cout << i << std::endl;
}

Parentheses around parameter names are optional, so the following is a semantically identical spelling of the declaration:

Gadget g(Widget i);

As a result, this program is well-formed and prints only 3 as output, because no Gadget or Widget objects are constructed.

Compliant Solution

This compliant solution demonstrates two equally-compliant ways to write the declaration of g. The first declaration, g1, uses an extra set of parenthesis around the argument to the constructor call, forcing the compiler to parse this as a local variable declaration of type Gadget instead of as a function declaration. The second declaration, g2, uses direct initialization to similar effect.

#include <iostream>

struct Widget {
  explicit Widget(int I) { std::cout << "Widget constructed" << std::endl; }
};

struct Gadget {
  explicit Gadget(Widget wid) { std::cout << "Gadget constructed" << std::endl; }
};

void f() {
  int i = 3;
  Gadget g1((Widget(i))); // Use extra parenthesis
  Gadget g2{Widget(i)}; // Use direct initialization
  std::cout << i << std::endl;
}

Running this program produces the expected output:

Widget constructed
Gadget constructed
Widget constructed
Gadget constructed
3

Risk Assessment

Syntactically ambiguous declarations can lead to unexpected program execution. However, it is likely that rudimentary testing would uncover violations of this rule.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography