在 C++ 中,不是在成员初始化列表中构造的类的任何成员都是在执行包含类的构造函数之前默认构造的。但是,如果该成员变量只是要在它所在的类的构造函数中构造,这似乎是非常浪费的。
我在下面提供了一个示例来阐明我的意思。这里,Example
类有一个类型为 LargeIntimidatingClass
的成员变量 x
。使用成员初始化列表(Example
中的第一个构造函数)x
仅构造一次。然而,如果x
不能使用成员初始化列表合理构造,它会被构造两次!
//This class used as part of the example class further below
class LargeIntimidatingClass {
// ...
//many member variables and functions
// ...
LargeIntimidatingClass() {
//Painfully expensive default initializer
}
LargeIntimidatingClass(int a, double b) {
//Complicated calculations involving a and b
}
};
//Here, this class has a LargeIntimidatingClass as a member variable.
class Example {
LargeIntimidatingClass x;
char c;
//Basic member initialization list constructor. Efficient!
Example(int a, double b, char c) : x(a,b), c(c) {}
//What if the parameters to the LargeIntimidatingClass's constructor
//need to be computed inside the Example's constructor beforehand?
Example(std::string sophisticatedArgument) {
//Oh no! x has already been default initialized (unnecessarily!)
int a = something1(sophisticatedArgument);
double b = something2(sophisticatedArgument);
//x gets constructed again! Previous (default) x is totally wasted!
x = LargeIntimidatingClass(a,b);
c = something3(sophisticatedArgument);
}
};
是的,我知道在这个愚蠢的例子中你可以写成 Example(string s) : x(f1(s),f2(s)), c(f3(s)) {}
,但我相信您可以想象这样一种情况,将一堆逻辑插入成员初始化列表是很麻烦的(甚至是不可能的)。
是否可以在成员初始化列表中未列出成员变量时禁用其默认构造函数?
最佳答案
您不能禁用构造。在到达构造函数的主体之前,必须初始化所有类成员。也就是说,您可以轻松解决该问题。您可以添加一个私有(private)静态成员函数,它获取 a
和 b
并从中返回一个 LargeIntimidatingClass
class Example {
LargeIntimidatingClass x;
char c;
static LargeIntimidatingClass make_LargeIntimidatingClass(std::string sophisticatedArgument)
{
int a = something1(sophisticatedArgument);
double b = something2(sophisticatedArgument);
return LargeIntimidatingClass(a,b);
}
static char make_c(std::string sophisticatedArgument)
{
return something3(sophisticatedArgument);
}
public:
//Basic member initialization list constructor. Efficient!
Example(int a, double b, char c) : x(a,b), c(c) {}
// now we use helpers to initialize in the member initialization list
Example(std::string sophisticatedArgument) : x(make_LargeIntimidatingClass(sophisticatedArgument), c(make_c(sophisticatedArgument) {
//now everything is initialized correctly
}
};
关于c++ - 在构造函数代码之前禁用默认类成员初始化,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/53962686/