接受多个参数的显式构造函数

使一个构造函数具有多个参数 explicit有任何(有用的)效果吗？

例如:

class A {
public:
explicit A( int b, int c ); // does explicit have any (useful) effect?
};
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You'd stumble upon it for brace initialization (for instance in arrays)

struct A {
explicit A( int b, int c ) {}
};


struct B {
B( int b, int c ) {}
};


int main() {
B b[] = \{\{1,2}, {3,5}}; // OK


A a1[] = {A{1,2}, A{3,4}}; // OK


A a2[] = \{\{1,2}, {3,4}}; // Error


return 0;
}
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最佳答案

Up until C++11, yeah, no reason to use explicit on a multi-arg constructor.

That changes in C++11, because of initializer lists. Basically, copy-initialization (but not direct initialization) with an initializer list requires that the constructor not be marked explicit.

Example:

struct Foo { Foo(int, int); };
struct Bar { explicit Bar(int, int); };


Foo f1(1, 1); // ok
Foo f2 {1, 1}; // ok
Foo f3 = {1, 1}; // ok


Bar b1(1, 1); // ok
Bar b2 {1, 1}; // ok
Bar b3 = {1, 1}; // NOT OKAY
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The excellent answers by @StoryTeller and @Sneftel are the main reason. However, IMHO, this makes sense (at least I do it), as part of future proofing later changes to the code. Consider your example:

class A {
public:
explicit A( int b, int c );
};

This code doesn't directly benefit from explicit.

Some time later, you decide to add a default value for c, so it becomes this:

class A {
public:
A( int b, int c=0 );
};

When doing this, you're focussing on the c parameter - in retrospect, it should have a default value. You're not necessarily focussing on whether A itself should be implicitly constructed. Unfortunately, this change makes explicit relevant again.

So, in order to convey that a ctor is explicit, it might pay to do so when first writing the method.

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Here's my five cents to this discussion:

struct Foo {
Foo(int, double) {}
};


struct Bar {
explicit Bar(int, double) {}
};


void foo(const Foo&) {}
void bar(const Bar&) {}


int main(int argc, char * argv[]) {
foo({ 42, 42.42 }); // valid
bar({ 42, 42.42 }); // invalid
return 0;
}

As you can easily see, explicit prevents from using initializer list alongside with bar function bacause the constructor of struct Bar is declared as explicit.


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