What are: Attribute Specifier Sequences in C++?

🗞️ Important for: Detecting errors, Optimizing the code and Providing additional information to the developer.

What are: Attribute Specifier Sequences in C++?

Attribute Specifiers in C++ are metadata that provide additional information to the compiler about how it should treat certain parts of the code.

These attributes can help:

  • Detect errors;
  • Optimize the code;
  • or Provide additional information to the developer.

Let’s see some examples of attributes in C++.

Examples of Attributes in C++

01. [[nodiscard]]:

This attribute indicates that the return value of a function should not be ignored. If the return value is ignored, the compiler generates a warning.

[[nodiscard]] int computeValue() {
 return 42;

int main() {
 computeValue(); // Compiler warning: return value ignored
 int result = computeValue(); // Correct, the return value is used
 return 0;

The compiler will issue a warning if the computeValue function is called and its return value is not used. This is useful to avoid accidentally losing important valuables.

If you remove [[nodiscard]] from the code note that there will be no warnings, but with [[nodiscard]] the output will be similar to this:

nodiscard.cpp: In function ‘int main()’:
nodiscard.cpp:8:17: warning: ignoring return value of ‘int computeValue()’, declared with attribute ‘nodiscard’ [-Wunused-result]
 8 | computeValue(); // Compiler warning: return value ignored
 | ~~~~~~~~~~~~~^~
nodiscard.cpp:3:19: note: declared here
 3 | [[nodiscard]] int computeValue() {

02. [[fallthrough]]:

This attribute is used in a case statement within a switch to indicate that the fallthrough to the next case is intentional. This helps avoid compiler warnings about accidental crashes.

void process(int value) {
 switch(value) {
 case 1:
 // Code for case 1
 case 2:
 // Code for case 2
 // Code for other cases

In the switch example, [[fallthrough]] indicates that the fallthrough from case 1 to case 2 is intentional. Without this attribute, the compiler could issue a warning about the lack of break in case 1.

03. [[unlikely]]:

This attribute suggests to the compiler that the probability of executing a branch of code is low. This can help in optimizing the generated code.

#include <iostream>

void checkValue(int value) {
 if (value == 42) [[unlikely]] {
 std::cout << "The value is 42, which is unlikely!\n";
 } else {
 std::cout << "The value is not 42.\n";

int main() {
 return 0;

The compiler can optimize the generated code based on the hint that the condition value == 42 is unlikely to be true, possibly reordering the code to improve performance in the most common case.

These attributes are part of the ongoing modernization of the C++ language to make code more expressive and help with early error detection as well as performance optimization.

For more information visit cpp reference.

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Marcos Oliveira

Marcos Oliveira

Software developer

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