我有一些模板代码可以实现非常繁重的计算，但我只需要它用于浮点数和双精度数。目标是模板实例化只在一个编译单元中完成一次，而不是对每个文件重复。
我尝试按照以下Stackoverflow帖子中的提示进行操作：
和类似的重复问题。我想出了以下测试来说明问题：
A.H
#pragma once
#include <cmath>
template<typename T>
struct A
{
static T foo(T a, T b)
{
//do some heavy computations
T v1 = pow(a, b);
return pow(v1, b);
}
};
//explicit template instantiations, the declaration
extern template struct A<float>;
extern template struct A<double>;
A.cpp
#include "A.h"
//explicit template instantiations, the definition
template struct A<float>;
template struct A<double>;
Main.cpp的
#include "A.h"
int main()
{
//use A
float result = A<float>::foo(0, 0);
return (int)result; //return it so that it doesn't get optimized away
}
当我现在查看生成的.obj文件（dumpbin / DISASM）时，我得到以下输出：
A.OBJ
Dump of file A.obj
File Type: COFF OBJECT
?foo@?$A@M@@SAMMM@Z (public: static float __cdecl A<float>::foo(float,float)):
0000000000000000: F3 0F 11 4C 24 10 movss dword ptr [rsp+10h],xmm1
0000000000000006: F3 0F 11 44 24 08 movss dword ptr [rsp+8],xmm0
000000000000000C: 55 push rbp
000000000000000D: 57 push rdi
000000000000000E: 48 81 EC 18 01 00 sub rsp,118h
00
0000000000000015: 48 8D 6C 24 30 lea rbp,[rsp+30h]
000000000000001A: 48 8B FC mov rdi,rsp
000000000000001D: B9 46 00 00 00 mov ecx,46h
0000000000000022: B8 CC CC CC CC mov eax,0CCCCCCCCh
0000000000000027: F3 AB rep stos dword ptr [rdi]
0000000000000029: F3 0F 10 8D 08 01 movss xmm1,dword ptr [rbp+108h]
00 00
0000000000000031: F3 0F 10 85 00 01 movss xmm0,dword ptr [rbp+100h]
00 00
0000000000000039: E8 00 00 00 00 call ?pow@@YAMMM@Z
000000000000003E: F3 0F 11 45 04 movss dword ptr [rbp+4],xmm0
0000000000000043: F3 0F 10 8D 08 01 movss xmm1,dword ptr [rbp+108h]
00 00
000000000000004B: F3 0F 10 45 04 movss xmm0,dword ptr [rbp+4]
0000000000000050: E8 00 00 00 00 call ?pow@@YAMMM@Z
0000000000000055: 48 8D A5 E8 00 00 lea rsp,[rbp+0E8h]
00
000000000000005C: 5F pop rdi
000000000000005D: 5D pop rbp
000000000000005E: C3 ret
?foo@?$A@N@@SANNN@Z (public: static double __cdecl A<double>::foo(double,double)):
0000000000000000: F2 0F 11 4C 24 10 movsd mmword ptr [rsp+10h],xmm1
0000000000000006: F2 0F 11 44 24 08 movsd mmword ptr [rsp+8],xmm0
000000000000000C: 55 push rbp
000000000000000D: 57 push rdi
000000000000000E: 48 81 EC 18 01 00 sub rsp,118h
00
0000000000000015: 48 8D 6C 24 30 lea rbp,[rsp+30h]
000000000000001A: 48 8B FC mov rdi,rsp
000000000000001D: B9 46 00 00 00 mov ecx,46h
0000000000000022: B8 CC CC CC CC mov eax,0CCCCCCCCh
0000000000000027: F3 AB rep stos dword ptr [rdi]
0000000000000029: F2 0F 10 8D 08 01 movsd xmm1,mmword ptr [rbp+108h]
00 00
0000000000000031: F2 0F 10 85 00 01 movsd xmm0,mmword ptr [rbp+100h]
00 00
0000000000000039: E8 00 00 00 00 call pow
000000000000003E: F2 0F 11 45 08 movsd mmword ptr [rbp+8],xmm0
0000000000000043: F2 0F 10 8D 08 01 movsd xmm1,mmword ptr [rbp+108h]
00 00
000000000000004B: F2 0F 10 45 08 movsd xmm0,mmword ptr [rbp+8]
0000000000000050: E8 00 00 00 00 call pow
0000000000000055: 48 8D A5 E8 00 00 lea rsp,[rbp+0E8h]
00
000000000000005C: 5F pop rdi
000000000000005D: 5D pop rbp
000000000000005E: C3 ret
....
Main.obj
Dump of file Main.obj
File Type: COFF OBJECT
?foo@?$A@M@@SAMMM@Z (public: static float __cdecl A<float>::foo(float,float)):
0000000000000000: F3 0F 11 4C 24 10 movss dword ptr [rsp+10h],xmm1
0000000000000006: F3 0F 11 44 24 08 movss dword ptr [rsp+8],xmm0
000000000000000C: 55 push rbp
000000000000000D: 57 push rdi
000000000000000E: 48 81 EC 18 01 00 sub rsp,118h
00
0000000000000015: 48 8D 6C 24 30 lea rbp,[rsp+30h]
000000000000001A: 48 8B FC mov rdi,rsp
000000000000001D: B9 46 00 00 00 mov ecx,46h
0000000000000022: B8 CC CC CC CC mov eax,0CCCCCCCCh
0000000000000027: F3 AB rep stos dword ptr [rdi]
0000000000000029: F3 0F 10 8D 08 01 movss xmm1,dword ptr [rbp+108h]
00 00
0000000000000031: F3 0F 10 85 00 01 movss xmm0,dword ptr [rbp+100h]
00 00
0000000000000039: E8 00 00 00 00 call ?pow@@YAMMM@Z
000000000000003E: F3 0F 11 45 04 movss dword ptr [rbp+4],xmm0
0000000000000043: F3 0F 10 8D 08 01 movss xmm1,dword ptr [rbp+108h]
00 00
000000000000004B: F3 0F 10 45 04 movss xmm0,dword ptr [rbp+4]
0000000000000050: E8 00 00 00 00 call ?pow@@YAMMM@Z
0000000000000055: 48 8D A5 E8 00 00 lea rsp,[rbp+0E8h]
00
000000000000005C: 5F pop rdi
000000000000005D: 5D pop rbp
000000000000005E: C3 ret
....
A::foo按预期在A.obj中实例化。但是代码也会再次输入Main.obj，完全忽略了extern关键字。
如何告诉编译器（Visual Studio 2017，发布模式）不要内联方法，而是使用A.obj中的版本？
答案 0 :(得分：3)
您可以使用__declspec(noinline)执行此操作。
但内联版本可能会更快。如果您担心二进制文件大小，您的.exe文件将只有该函数的单个实例。来自A.obj的代码未使用，并且在死代码消除步骤中将被链接器丢弃。
更新：将此信息放入A.h：
static __declspec( noinline ) T foo( T a, T b )
{
//do some heavy computations
T v1 = pow( a, b );
return pow( v1, b );
}
我使用Visual C ++ 2017 15.6.7版本32和64位构建，对于这两个平台，Main.cpp编译为：
; Line 5
call ?foo@?$A@M@@SAMMM@Z ; A<float>::foo
; Line 6
cvttss2si eax, xmm0
但是，如果你这样做是为了减少编译时间，我不确定noinline会有所帮助。而是从A.h（离开声明）中删除函数体，将其移动到A.cpp中。理想情况下，还要从A.h中删除特征头（或保留最小的定义数据结构的头），并将特征头包含在A.cpp中。