您不能使用类数组,但可以使用指向函数的指针数组.

typedef std::unique_ptr (*Creator)();

template

std::unique_ptr make() { return new T{}; }

Creator const array[] = { make, make, make };

int main() {

std::unique_ptr b = array[1]();

b->foo();

}

对于那些担心创建这么多模板函数的成本的人来说,这是一个例子：

#include

struct Base { virtual void foo() const = 0; };

struct A: Base { void foo() const { printf("A"); } };

struct B: Base { void foo() const { printf("B"); } };

struct C: Base { void foo() const { printf("C"); } };

typedef Base* (*Creator)();

template

static Base* make() { return new T{}; }

static Creator const array[] = { make, make, make };

Base* select_array(int i) {

return array[i]();

}

Base* select_switch(int i) {

switch(i) {

case 0: return make();

case 1: return make();

case 2: return make();

default: return 0;

}

}

LLVM / Clang生成以下输出：

define %struct.Base* @select_array(int)(i32 %i) uwtable {

%1 = sext i32 %i to i64

%2 = getelementptr inbounds [3 x %struct.Base* ()*]* @array, i64 0, i64 %1

%3 = load %struct.Base* ()** %2, align 8, !tbaa !0

%4 = tail call %struct.Base* %3()

ret %struct.Base* %4

}

define noalias %struct.Base* @select_switch(int)(i32 %i) uwtable {

switch i32 %i, label %13 [

i32 0, label %1

i32 1, label %5

i32 2, label %9

]

; :1 ; preds = %0

%2 = tail call noalias i8* @operator new(unsigned long)(i64 8)

%3 = bitcast i8* %2 to i32 (...)***

store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* @vtable for A, i64 0, i64 2) to i32 (...)**), i32 (...)*** %3, align 8

%4 = bitcast i8* %2 to %struct.Base*

br label %13

; :5 ; preds = %0

%6 = tail call noalias i8* @operator new(unsigned long)(i64 8)

%7 = bitcast i8* %6 to i32 (...)***

store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* @vtable for B, i64 0, i64 2) to i32 (...)**), i32 (...)*** %7, align 8

%8 = bitcast i8* %6 to %struct.Base*

br label %13

; :9 ; preds = %0

%10 = tail call noalias i8* @operator new(unsigned long)(i64 8)

%11 = bitcast i8* %10 to i32 (...)***

store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* @vtable for C, i64 0, i64 2) to i32 (...)**), i32 (...)*** %11, align 8

%12 = bitcast i8* %10 to %struct.Base*

br label %13

; :13 ; preds = %9, %5, %1, %0

%.0 = phi %struct.Base* [ %12, %9 ], [ %8, %5 ], [ %4, %1 ], [ null, %0 ]

ret %struct.Base* %.0

}

不幸的是,它不够智能,无法使用常规数组代码自动内联函数(LLVM优化器的已知问题,我不知道gcc是否做得更好)……但是使用switch确实是可能的.