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C++ attributes

C++ attributes nodiscard noreturn assume noexcept

Table of contents

Brief
Attribute [[nodiscard]]
noexcept
Error output
Compilation output
References

Brief

[[nodiscard]]
[[noreturn]]
[[carries_dependency]]
[[assume]]
noexcept

Attribute `[[nodiscard]]`

Used to indicate when it is an error to ignore a return value from a function
Can be applied to constructors as of C++20
Can have a message to explain the error
- [[nodiscard("Lock objects should never be discarted")]]
Should be used extensively.
Any non-mutating (getter/accessor/{eonse) function should be `[[nodiscard]]
cos()
vector::insert(): Return an iterator that points to the first of the newly inserted elements.
Can be checked / enforced with static analysis

☢ Warning

filename.cpp:46:3: warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]

Function

[[nodiscard]] int get_zero() { return 0; }

[[nodiscard("Some custom message")]] int get_one() { return 1; }

[[nodiscard("XXX find me XXX on warning messages")]] int unused_function() { return -1; }

void testFunction() {
  fmt::print(stderr, "testFunction()\n");
  get_zero();
  get_one();
}

Types

struct [[nodiscard]] ErrorType {};
ErrorType get_two() { return {}; }

void testType() {
  fmt::print(stderr, "testType()\n");
  get_two();
}

Lambda

void testLambda() {
  fmt::print(stderr, "testLambda()\n");
  auto lm = [] [[nodiscard]] () -> int { return 42; };
  lm(); // would emit a warning
}

Constructor

struct FDHolder {
  [[nodiscard]] FDHolder(int FD) {};
  FDHolder() {};
};

void testConstructor() {
  fmt::print(stderr, "testConstructor()\n");
  FDHolder{42};   // warning
  FDHolder h{42}; // constructed object not discarded, no warning
  FDHolder{};     // defalt constructed, no warning
}

`noexcept`

noexcept notifies the user (and compiler) that a function may not throw an exception. If an exception is thrown from that function, terminate MUST be called.

void testNoExcept() noexcept {
  fmt::print(stderr, "testNoExcept()\n");
  // required to terminate the program
  throw 42;
}

  try {
    testFunction();
    testLambda();
    testConstructor();
    testNoExcept(); // terminate is called here
    testFunction(); // never called
  } catch (...) {
    // catch is irrelevant, `terminate` is called
  }
  fmt::print(stderr, "End of program\n");

Error output

testFunction()
testLambda()
testConstructor()
testNoExcept()
terminate called after throwing an instance of 'int'

Compilation output

attributes.cpp: In function ‘void testFunction()’:
attributes.cpp:60:11: warning: ignoring return value of ‘int get_zero()’, declared with attribute ‘nodiscard’ [-Wunused-result]
   60 |   get_zero();
      |   ~~~~~~~~^~
attributes.cpp:52:19: note: declared here
   52 | [[nodiscard]] int get_zero() { return 0; }
      |                   ^~~~~~~~
attributes.cpp:61:10: warning: ignoring return value of ‘int get_one()’, declared with attribute ‘nodiscard’: ‘Some custom message’ [-Wunused-result]
   61 |   get_one();
      |   ~~~~~~~^~
attributes.cpp:54:42: note: declared here
   54 | [[nodiscard("Some custom message")]] int get_one() { return 1; }
      |                                          ^~~~~~~
attributes.cpp: In function ‘void testType()’:
attributes.cpp:73:10: warning: ignoring returned value of type ‘ErrorType’, declared with attribute ‘nodiscard’ [-Wunused-result]
   73 |   get_two();
      |   ~~~~~~~^~
attributes.cpp:69:11: note: in call to ‘ErrorType get_two()’, declared here
   69 | ErrorType get_two() { return {}; }
      |           ^~~~~~~
attributes.cpp:68:22: note: ‘ErrorType’ declared here
   68 | struct [[nodiscard]] ErrorType {};
      |                      ^~~~~~~~~
attributes.cpp: In function ‘void testLambda()’:
attributes.cpp:83:5: warning: ignoring return value of ‘testLambda()::<lambda()>’, declared with attribute ‘nodiscard’ [-Wunused-result]
   83 |   lm(); // would emit a warning
      |   ~~^~
attributes.cpp:82:13: note: declared here
   82 |   auto lm = [] [[nodiscard]] () -> int { return 42; };
      |             ^
attributes.cpp: In constructor ‘FDHolder::FDHolder(int)’:
attributes.cpp:91:30: warning: unused parameter ‘FD’ [-Wunused-parameter]
   91 |   [[nodiscard]] FDHolder(int FD) {};
      |                          ~~~~^~
attributes.cpp: In function ‘void testConstructor()’:
attributes.cpp:97:15: warning: ignoring return value of ‘FDHolder::FDHolder(int)’, declared with attribute ‘nodiscard’ [-Wunused-result]
   97 |   FDHolder{42};   // warning
      |               ^
attributes.cpp:91:17: note: declared here
   91 |   [[nodiscard]] FDHolder(int FD) {};
      |                 ^~~~~~~~
attributes.cpp: In function ‘void testNoExcept()’:
attributes.cpp:113:3: warning: ‘throw’ will always call ‘terminate’ [-Wterminate]
  113 |   throw 42;
      |   ^~~~~~~~