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clang-tools-extra/docs/clang-tidy/checks/modernize-use-inline-const-variables-in-headers.rst

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.. title:: clang-tidy - modernize-use-inline-const-variables-in-headers
modernize-use-inline-const-variables-in-headers
===============================================
Suggests switching to C++17 ``inline variables`` for non-inline const
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...const variable definitions...const variable declarations

LegalizeAdulthood: ...`const variable definitions`...`const variable declarations`
variable definitions and extern const variable declarations in header files.
Non-inline const variables make a separate instance of the variable for each
translation unit that includes the header, which can lead to subtle violation
of the ODR. Extern const variables are a deprecated way to define a constant
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...variables are a deprecated way...

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since C++17.
.. code-block:: c++
// Foo.h
const int ConstFoo = 1; // Warning: should be marked as 'inline'
namespace N {
constexpr int NamespaceFoo = 1; // Warning: should be marked as 'inline'
}
extern const int ExternFoo; // Warning: should be converted to C++17 'inline variable'
struct S {
static const int StructFoo = 1; // OK: the variable is not at file scope
};
int NonConstFoo = 1; // No warning: non-const global variables have external linkage
const volatile int VolatileFoo = 1; // No warning: volatile global variables have external linkage
template<typename T>
const auto TemplateFoo = sizeof(T); // OK: templates and their instantiations have external linkage
inline const int InlineFoo = 1; // no warning: already fixed
// No warning: C has no 'inline variables'
extern "C" {
const int CFoo0 = 1;
}
extern "C" const int CFoo1 = 1;
// No warning: 'inline' is invisible when within an unnamed namespace
namespace {
const int AnonNamespaceFoo = 1;
}
Options
-------
.. option:: HeaderFileExtensions
A comma-separated list of filename extensions of header files (the filename
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Please highlight option values in this section with single back-ticks.

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extensions should not include "." prefix). Default is `h,hh,hpp,hxx`.
For header files without an extension, use an empty string (if there are no
other desired extensions) or leave an empty element in the list. E.g.,
`h,hh,hpp,hxx,` (note the trailing comma).
.. option:: CheckNonInline
When `true`, the check will find non-inline const variable definitions in
header files and suggest fixes for them. Default is `true`.
.. option:: CheckExtern
When `true`, the check will find extern const variable declarations in
header files. But the checker won't fix it as the definition may be located
in another translation unit. Default is `true`.