问题描述
在Memory.h中,我有:
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<table style="background:#c3c3c3" id="t_1">
<tbody>
<tr class="amenities-row" id="ar_1">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_1" id="am_1" value="1" disabled>
<label for="am_1">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_1" type="checkbox" value="1" checked>
<label for="checkbox_am_1">
</label>
</div>
</td>
<td>
Amenity 1
</td>
</tr>
<tr class="amenities-row" id="ar_2">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_1" id="am_2" value="1" disabled>
<label for="am_2">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_2" type="checkbox" value="1" checked>
<label for="checkbox_am_2">
</label>
</div>
</td>
<td>
Amenity 2
</td>
</tr>
<tr class="amenities-row" id="ar_3">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_1" id="am_3" value="1">
<label for="am_3">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_3" type="checkbox" value="1" checked>
<label for="checkbox_am_3">
</label>
</div>
</td>
<td>
Amenity 3
</td>
</tr>
<tr class="amenities-row" id="ar_4">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_1" id="am_4" value="1" disabled>
<label for="am_4">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_4" type="checkbox" value="1" checked>
<label for="checkbox_am_4">
</label>
</div>
</td>
<td>
Amenity 4
</td>
</tr>
</tbody>
</table>
<table id="t_2">
<tbody>
<tr class="amenities-row" id="ar_5">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_2" id="am_5" value="1">
<label for="am_5">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_5" type="checkbox" value="1" checked>
<label for="checkbox_am_5">
</label>
</div>
</td>
<td>
Amenity 5
</td>
</tr>
<tr class="amenities-row" id="ar_6">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_2" id="am_6" value="1">
<label for="am_6">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_6" type="checkbox" value="1" checked>
<label for="checkbox_am_6">
</label>
</div>
</td>
<td>
Amenity 6
</td>
</tr>
<tr class="amenities-row" id="ar_7">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_2" id="am_7" value="1" disabled>
<label for="am_7">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_7" type="checkbox" value="1" checked>
<label for="checkbox_am_7">
</label>
</div>
</td>
<td>
Amenity 7
</td>
</tr>
<tr class="amenities-row" id="ar_8">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_2" id="am_8" value="1" disabled>
<label for="am_8">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_8" type="checkbox" value="1" checked>
<label for="checkbox_am_8">
</label>
</div>
</td>
<td>
Amenity 8
</td>
</tr>
</tbody>
</table>
<table id="t_3" style="background:lime">
<tbody>
<tr class="amenities-row" id="ar_9">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_3" id="am_9" value="1" disabled>
<label for="am_9">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_9" type="checkbox" value="1" checked>
<label for="checkbox_am_9">
</label>
</div>
</td>
<td>
Amenity 9
</td>
</tr>
<tr class="amenities-row" id="ar_10">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_3" id="am_10" value="1" disabled>
<label for="am_10">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_10" type="checkbox" value="1" checked>
<label for="checkbox_am_10">
</label>
</div>
</td>
<td>
Amenity 10
</td>
</tr>
<tr class="amenities-row" id="ar_11">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_3" id="am_11" value="1" disabled>
<label for="am_11">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_11" type="checkbox" value="1" checked>
<label for="checkbox_am_11">
</label>
</div>
</td>
<td>
Amenity 11
</td>
</tr>
<tr class="amenities-row" id="ar_12">
<td>
<div class="radio radio-info pl-2">
<input type="radio" name="base_cat_3" id="am_12" value="1" disabled>
<label for="am_12">
</label>
</div>
</td>
<td>
<div class="checkbox checkbox-info pl-2">
<input name="selected_am" id="checkbox_am_12" type="checkbox" value="1" checked>
<label for="checkbox_am_12">
</label>
</div>
</td>
<td>
Amenity 12
</td>
</tr>
</tbody>
</table>
在main.cpp中,我有:
#pragma once
class Memory
{public:
template <typename T,typename ... TArgs>
static T* newAlloc(TArgs ... args)
{
return new T(args ...);
}
};
即使在我粘贴Memory.h之后才完全定义Foo,这也可以正常编译。我对为什么编译感到困惑。编译器粘贴到Memory.h之后的等效代码不是真的吗,就像main.cpp中这样:
#include "Memory.h" // The template class is defined here
class Foo
{public:
Foo(int a,const char* c) {}
};
int main()
{
Memory::newAlloc<Foo>(7,"str");
}
???由于无法编译,因此出现错误:
找不到'newAlloc'标识符'newAlloc'不是以下成员 记忆
基本上,它需要Foo的完整定义。如果我将Foo类定义放在Memory 1之上,则可以正常编译。因此,考虑到我的第二个版本等效于模板实例化之后的代码(是吗?),那么为什么第一个版本可以编译而第二个版本不能编译?
我正在使用Visual Studio 2019 16.4.5
解决方法
在编译器粘贴到Memory.h中后,等效代码是否正确,就像main.cpp中这样:
不,这不是事实。这将是等效的:
class Memory
{public:
template <typename T,typename ... TArgs>
static T* newAlloc(TArgs ... args)
{
return new T(args ...);
}
};
class Foo
{public:
Foo(int a,const char* c) {}
};
int main()
{
Memory::newAlloc<Foo>(7,"str");
}
您会发现它可以编译。
为什么第一个版本会编译
之所以有效,是因为Foo
是在模板以Foo
作为模板参数实例化之前定义的。
即使Foo直到我粘贴到Memory.h之后才完全定义,它仍可以正常编译
是的,因为在这里实例化了Memory::newAlloc
时:
Memory::newAlloc(7,"str");
Foo
的定义已经存在,因此可以很好地编译。
在编译器粘贴到Memory.h中后,等效代码是否正确,就像main.cpp中这样:
在这种情况下,不在此行上:
static Foo* newAlloc(int a,const char* c)
由于尚未定义Foo
,因此代码将无法编译。