In-class member initializer using a constructor: is it allowed?

Question

I recently found an interesting piece of code in the article Get to Know the New C++11 Initialization Forms by Danny Kalev:

class C
{
string s("abc");
double d=0;
char * p {nullptr};
int y[5] {1,2,3,4};
public:
C();
};

The line string s("abc"); seems suspicious to me. I thought that using a constructor is not allowed while a member is initialized in-class. And this code (simplified to class C { string s("abc"); };`) doesn't compile with

clang 3.6.1 (compiler arguments are -std=c++11 -Wall -Wextra -Werror -pedantic-errors)
g++ 5.1.0 (compiler arguments are the same: -std=c++11 -Wall -Wextra -Werror -pedantic-errors)
vc++ 18.00.21005.1 (compiler arguments are /EHsc /Wall /wd4514 /wd4710 /wd4820 /WX /Za)
vc++ 19.00.22929.0 (compiler arguments are predefined by the service: /EHsc /nologo /W4 /c)

Am I right and there is an error in this article?

score 4 · Accepted Answer · answered Jun 05 '15 at 16:29

4

Am I right and there is an error in this article?

Yes, it is an error in the article.

Only a brace-or-equal-initializer is allowed in the declaration of a data member. The initializations of d, p, and y are correct, but not s. The rationale for this is that using an expression-list would ambiguate the declaration with a function declaration and it would also cause conflicts with name lookup in the class body.

answered Jun 05 '15 at 16:29

David G

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Thank you. Could you provide a reference to the standard? I think that you pointed out the true rationale for this behavior. – Constructor Jun 05 '15 at 16:34
@Constructor The standard doesn't explicitly give a rationale. The rule is simply found in [class.mem]/p4. Also, http://stackoverflow.com/a/24837330/701092 – David G Jun 05 '15 at 16:43
Do you mean [class.mem]/p5? And thank you very much for this reference! It is very good. – Constructor Jun 05 '15 at 16:46
@Constructor I'm using the latest draft, n4431. You're probably using n3242. – David G Jun 05 '15 at 16:52
Oh, yes. n3337, to be precise. – Constructor Jun 05 '15 at 16:53
I usually use nearest to C++11 standard drafts to check pure C++11 features. – Constructor Jun 05 '15 at 17:04

score 1 · Answer 2 · edited Jun 05 '15 at 17:17

1

An example from Bjarne Stroustrup:

class A {
    public:
        A() {}
        A(int a_val) : a(a_val) {}
        A(D d) : b(g(d)) {}
        int a = 7;
        int b = 5;  
    private:
        HashingFunction hash_algorithm{"MD5"};  // Cryptographic hash to be applied to all A instances
        std::string s{"Constructor run"};       // String indicating state in object lifecycle
    };

edited Jun 05 '15 at 17:17

answered Jun 05 '15 at 16:42

pr3sto

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Thank you for your reference. – Constructor Jun 05 '15 at 16:52

In-class member initializer using a constructor: is it allowed?

2 Answers2