如何使用函數初始化靜態常量成員

Question

在一個非常簡化的情況下，我有以下設置，其中，我只是想從類A的函數（單例和實例無關）初始化一個常量靜態成員（類foo）對這個問題）：

class A 
{ 
public: 

    static A instance; 
    A & getInstance() { return instance; } 
    int i(){ return 10;} 
    int j(){ return 20;} 
}; 

class foo { 
public: 

    static const int ii = A::getInstance().i() * A::getInstance().j(); 
}; 
const int foo::ii; 

int main() 
{ 
    foo f; 
    return 1; 
} 

目的是使用上面的函數初始化成員ii。但它會產生以下錯誤：

$ c++ static_constant.cpp 
static_constant.cpp:14:30: error: ‘A::getInstance()’ cannot appear in a constant-expression 
static_constant.cpp:14:42: error: a function call cannot appear in a constant-expression 
static_constant.cpp:14:44: error: ‘.’ cannot appear in a constant-expression 
static_constant.cpp:14:46: error: a function call cannot appear in a constant-expression 
static_constant.cpp:14:53: error: ‘A::getInstance()’ cannot appear in a constant-expression 
static_constant.cpp:14:65: error: a function call cannot appear in a constant-expression 
static_constant.cpp:14:67: error: ‘.’ cannot appear in a constant-expression 
static_constant.cpp:14:69: error: a function call cannot appear in a constant-expression 

請問你能幫我嗎？將欣賞它。

Answer 1

與代碼的幾個問題，但這裏有一個完整的編譯樣本：

class A 
{ 
public: 

    static A instance; 
    static A & getInstance() { return instance; } 
    int i(){ return 10;} 
    int j(){ return 20;} 
}; 

class foo { 
public: 

    static const int ii; 
}; 
const int foo::ii = A::getInstance().i() * A::getInstance().j(); 
A A::instance; 

Answer 2

您正在使用getInstance作爲靜態函數，但未聲明static。

變化getInstance聲明：

static A & getInstance() { return instance; }