17 Language support library [support]

17.8 Source location [support.srcloc]

17.8.1 Header <source_­location> synopsis [source.location.syn]

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The header <source_­location> defines the class source_­location that provides a means to obtain source location information.
namespace std { struct source_location; }

17.8.2 Class source_­location [support.srcloc.class]

17.8.2.1 General [support.srcloc.class.general]

namespace std { struct source_location { // source location construction static consteval source_location current() noexcept; constexpr source_location() noexcept; // source location field access constexpr uint_least32_t line() const noexcept; constexpr uint_least32_t column() const noexcept; constexpr const char* file_name() const noexcept; constexpr const char* function_name() const noexcept; private: uint_least32_t line_; // exposition only uint_least32_t column_; // exposition only const char* file_name_; // exposition only const char* function_name_; // exposition only }; }
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The type source_­location meets the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructible requirements ([utility.arg.requirements]).
Lvalues of type source_­location are swappable ([swappable.requirements]).
All of the following conditions are true:
  • (1.1)
    is_­nothrow_­move_­constructible_­v<source_­location>
  • (1.2)
    is_­nothrow_­move_­assignable_­v<source_­location>
  • (1.3)
    is_­nothrow_­swappable_­v<source_­location>
[Note 1:
The intent of source_­location is to have a small size and efficient copying.
It is unspecified whether the copy/move constructors and the copy/move assignment operators are trivial and/or constexpr.
— end note]
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The data members file_­name_­ and function_­name_­ always each refer to an ntbs.
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The copy/move constructors and the copy/move assignment operators of source_­location meet the following postconditions: Given two objects lhs and rhs of type source_­location, where lhs is a copy/move result of rhs, and where rhs_­p is a value denoting the state of rhs before the corresponding copy/move operation, then each of the following conditions is true:
  • (3.1)
    strcmp(lhs.file_­name(), rhs_­p.file_­name()) == 0
  • (3.2)
    strcmp(lhs.function_­name(), rhs_­p.function_­name()) == 0
  • (3.3)
    lhs.line() == rhs_­p.line()
  • (3.4)
    lhs.column() == rhs_­p.column()

17.8.2.2 Creation [support.srcloc.cons]

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static consteval source_location current() noexcept;
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Returns:
  • (1.1)
    When invoked by a function call whose postfix-expression is a (possibly parenthesized) id-expression naming current, returns a source_­location with an implementation-defined value.
    The value should be affected by #line ([cpp.line]) in the same manner as for __LINE__ and __FILE__.
    The values of the exposition-only data members of the returned source_­location object are indicated in Table 38.
    Table 38: Value of object returned by current [tab:support.srcloc.current]
    Element
    Value
    line_­
    A presumed line number ([cpp.predefined]).
    Line numbers are presumed to be 1-indexed; however, an implementation is encouraged to use 0 when the line number is unknown.
    column_­
    An implementation-defined value denoting some offset from the start of the line denoted by line_­.
    Column numbers are presumed to be 1-indexed; however, an implementation is encouraged to use 0 when the column number is unknown.
    file_­name_­
    A presumed name of the current source file ([cpp.predefined]) as an ntbs.
    function_­name_­
    A name of the current function such as in __func__ ([dcl.fct.def.general]) if any, an empty string otherwise.
  • (1.2)
    Otherwise, when invoked in some other way, returns a source_­location whose data members are initialized with valid but unspecified values.
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Remarks: Any call to current that appears as a default member initializer ([class.mem]), or as a subexpression thereof, should correspond to the location of the constructor definition or aggregate initialization that uses the default member initializer.
Any call to current that appears as a default argument ([dcl.fct.default]), or as a subexpression thereof, should correspond to the location of the invocation of the function that uses the default argument ([expr.call]).
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[Example 1: struct s { source_location member = source_location::current(); int other_member; s(source_location loc = source_location::current()) : member(loc) // values of member refer to the location of the calling function ([dcl.fct.default]) {} s(int blather) : // values of member refer to this location other_member(blather) {} s(double) // values of member refer to this location {} }; void f(source_location a = source_location::current()) { source_location b = source_location::current(); // values in b refer to this line } void g() { f(); // f's first argument corresponds to this line of code source_location c = source_location::current(); f(c); // f's first argument gets the same values as c, above } — end example]
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constexpr source_location() noexcept;
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Effects: The data members are initialized with valid but unspecified values.

17.8.2.3 Observers [support.srcloc.obs]

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constexpr uint_least32_t line() const noexcept;
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Returns: line_­.
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constexpr uint_least32_t column() const noexcept;
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Returns: column_­.
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constexpr const char* file_name() const noexcept;
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Returns: file_­name_­.
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constexpr const char* function_name() const noexcept;
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Returns: function_­name_­.