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{{cpp/language/attributes/title|assume|notes={{mark since c++23}}}}
{{cpp/language/attributes/navbar}}

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

===Syntax===
{{sdsc begin}}
{{sdsc|
{{ttb|{{c/core|[<!---->[}}assume{{c/core|(}}}} {{spar|expression}} {{ttb|{{c/core|)]<!---->]}}}}
}}
{{sdsc end}}

{{par begin}}
{{par|{{spar|expression}}|any expression (except unparenthesized [[cpp/language/operator other#Built-in comma operator|comma expressions]])}}
{{par end}}

===Explanation===
{{c/core|[[assume]]}} an only be applied to a [[cpp/language/statements#Expression statements|null statement]], as in {{c|[[assume(x > 0)]];}}. This statement is called an ''assumption''.

{{spar|expression}} is [[cpp/language/implicit conversion#Contextual conversions|contextually converted to {{c/core|bool}}]], but it is not evaluated (it is still [[cpp/language/expressions#Potentially-evaluated expressions|potentially evaluated]]).
* If the converted {{spar|expression}} would evaluate to {{c|true}} at the point where the assumption appears, the assumption has no effect.
* Otherwise, evaluation of the assumption has [[cpp/language/ub|runtime-undefined behavior]].

===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:

{{source
|code=
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
}}

===Example===
{{source
|code=
#include <cmath>

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);
}
}}

===Defect reports===
{{dr list begin}}
{{dr list item|wg=cwg|dr=2924|std=C++23|before=violating an assumption would result in undefined behavior|after=results in runtime-undefined behavior}}
{{dr list end}}

===References===
{{ref std c++23}}
{{ref std|section=9.12.3|title=Assumption attribute|id=dcl.attr.assume}}
{{ref std end}}

===See also===
{{dsc begin}}
{{dsc inc|cpp/utility/dsc unreachable}}
{{dsc inc|cpp/language/dsc contract_assert}}
{{dsc end}}

===External links===
{{elink begin}}
{{elink|Clang language extensions doc: [https://clang.llvm.org/docs/LanguageExtensions.html#langext-builtin-assume {{tt|__builtin_assume}}].}}
{{elink|Clang attribute reference doc: [https://clang.llvm.org/docs/AttributeReference.html#assume {{tt|assume}}].}}
{{elink|MSVC doc: [https://learn.microsoft.com/en-us/cpp/intrinsics/assume {{tt|__assume}}] built-in.}}
{{elink|GCC doc: [https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#index-assume-statement-attribute {{tt|__attribute__((assume(...)))}}].}}
{{elink end}}

{{langlinks|es|ja|ru|zh}}

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