C++ attribute: assume (since C++23) (original) (raw)

Specifies that the given expression is assumed to always evaluate to true at a given point in order to allow compiler optimizations based on the information given.

Contents

• 1 Syntax
• 2 Explanation
• 3 Notes
• 4 Example
• 5 Defect reports
• 6 References
• 7 See also
• 8 External links

[edit] Syntax

| | | | | ------------------------------------ | | | | [[assume( expression )]] | | | | | | |

[edit] Explanation

[[assume]] an only be applied to a null statement, as in [[assume(x > 0)]];. This statement is called an assumption.

expression is contextually converted to bool, but it is not evaluated (it is still potentially evaluated).

• If the converted expression would evaluate to true at the point where the assumption appears, the assumption has no effect.
• Otherwise, evaluation of the assumption has runtime-undefined behavior.

[edit] Notes

Since assumptions cause runtime-undefined behavior if they do not hold, they should be used sparingly.

One correct way to use them is to follow assertions with assumptions:

assert(x > 0); // trigger an assertion when NDEBUG is not defined and x > 0 is false [[assume(x > 0)]]; // provide optimization opportunities when NDEBUG is defined

[edit] Example

#include   void f(int& x, int y) { void g(int); void h();   [[assume(x > 0)]]; // Compiler may assume x is positive   g(x / 2); // More efficient code possibly generated   x = 3; int z = x;   [[assume((h(), x == z))]]; // Compiler may assume x would have the same value after // calling h // The assumption does not cause a call to h   h(); g(x); // Compiler may replace this with g(3);   h(); g(x); // Compiler may NOT replace this with g(3); // An assumption applies only at the point where it appears   z = std::abs(y);   [[assume((g(z), true))]]; // Compiler may assume g(z) will return   g(z); // Due to above and below assumptions, compiler may replace this with g(10);   [[assume(y == -10)]]; // Undefined behavior if y != -10 at this point   [[assume((x - 1) * 3 == 12)]];   g(x); // Compiler may replace this with g(5); }

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] References

• C++23 standard (ISO/IEC 14882:2024):

• 9.12.3 Assumption attribute [dcl.attr.assume]

[edit] See also

[edit] External links