<p>How to get the size of a non-static member of a class without construct that class?</p>
<h2 id="heading-c">C</h2>
<p><a target="_blank" href="https://en.cppreference.com/w/cpp/language/sizeof">sizeof operator</a>:</p>
<blockquote>
<p>When applied to an expression, sizeof does not evaluate the expression...</p>
</blockquote>
<p>Hence, we can write:</p>
<pre><code class="lang-cpp"><span class="hljs-keyword">sizeof</span>(((Foo*) <span class="hljs-number">0</span>)-&gt;m);
</code></pre>
<h2 id="heading-c">C++</h2>
<p><code>c++0x</code> allow that:</p>
<pre><code class="lang-cpp"><span class="hljs-comment">// a more elegant way</span>
<span class="hljs-keyword">sizeof</span>(Foo::m);
</code></pre>
<p><a target="_blank" href="https://timsong-cpp.github.io/cppwp/n4659/expr.prim.id.qual#2">expr.prim.id.qual</a>:</p>
<blockquote>
<p>A nested-name-specifier that denotes a class, optionally followed by the keyword template ([temp.names]), and then followed by the name of a member of either that class ([class.mem]) or one of its base classes, is a qualified-id; ... The result is an lvalue if the member is a static member function or a data member and a prvalue otherwise.</p>
</blockquote>
<h2 id="heading-reference">Reference</h2>
<p><a target="_blank" href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2253.html">Extending sizeof to apply to non-static data members without an object (revision 1)</a></p>
<p><a target="_blank" href="https://stackoverflow.com/questions/5976879/getting-the-size-of-member-variable">Getting the size of member variable</a></p>


How to get the size of a non-static member of a class without construct that class?

## C

[sizeof operator](https://en.cppreference.com/w/cpp/language/sizeof):

> When applied to an expression, sizeof does not evaluate the expression...

Hence, we can write:

```cpp
sizeof(((Foo*) 0)->m);
```

## C++

`c++0x` allow that:

```cpp
// a more elegant way
sizeof(Foo::m);
```

[expr.prim.id.qual](https://timsong-cpp.github.io/cppwp/n4659/expr.prim.id.qual#2):

> A nested-name-specifier that denotes a class, optionally followed by the keyword template ([temp.names]), and then followed by the name of a member of either that class ([class.mem]) or one of its base classes, is a qualified-id; ... The result is an lvalue if the member is a static member function or a data member and a prvalue otherwise.

## Reference

[Extending sizeof to apply to non-static data members without an object (revision 1)](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2253.html)

[Getting the size of member variable](https://stackoverflow.com/questions/5976879/getting-the-size-of-member-variable)

Size of a non-static member of a class

後端工程師。
不定時張貼一些寫扣時的筆記。


帕秋莉的魔法筆記

帕秋莉的魔法筆記

Size of a non-static member of a class

C

C++

Reference