C++ primer 5th exercise 7.52

Question

Exercise
Using our first version of Sales_data from § 2.6.1 (p. 72), explain the following initialization. Identify and fix any problems.

My code:

#include 
#include 
using namespace std;

struct Sales_data
{
    string bookNo;
    unsigned sold_units = 0;
    double revenue = 0.0;
};

int main()
{
    Sales_data item = { "978-1234567",25,15.99 };
    cout << item.bookNo << ' ' << item.sold_units << ' ' << item.revenue << endl;
    system("pause");
}

I first thought that the complier will give me an error, since there are in-class initial value to prevent "Sales_data" to be a aggregate class. But it does not.

After running the code, I think that the complier constructs a temp object using the initialization list, copy the temp object to "item", and finally destruct the temp object.

My question is: Is it because I code on Visual Studio 2017 that the complier doesn't send me an error? And what happened to the code above.

user_10871073 · Answer

Your confusion is caused by the fact that the use of initializers and initializer lists changed significantly between the C++98, C++11 and C++14 standards.

The code you have shown is badly-formed according to C++98 (the in-class initializers are not allowed) or C++11 (the brace-enclosed initializer list is not allowed), but is acceptable according to C++14 (or later).

The MSVC compiler in Visual Studio 2017 uses the C++14 Standard, by default. You can explicitly set it to use the C++11 Standard (IIRC – that option has been removed in VS 2019) if you want to be 'more compatible' with the Primer you are using. Select your project's "Properties → C/C++ → Language → C++ Language Standard" or add the /std:c++11 command-line switch.

Also, for strict(er) compliance with the selected Language Standard, it also advisable to select "Conformance Mode" (also in "Properties → C/C++ → Language", or via the /permissive- switch).

C++ primer 5th exercise 7.52

1 Answers