The features of C++17 were a big step in the history of C++ and brought a lot of new features. In C++17, there are new auto rules for direct-list-initialization. This feature in C++, changes the rules for auto deduction from a braced-init-list. In this post, we explain what these new rules for auto deduction with examples are.
Table of Contents
What is auto keyword in C++?
The auto keyword arrives with the new features in C++11 and improved in C++17, can be used as a placeholder type specifier (an auto-typed variable), or it can be used in function declaration, or in a structured binding declaration. The auto keyword can be used with other new CLANG standards like C++14, C++17, etc.
What are the new rules for auto deduction in C++ 17?
In C++17, For copy-list-initialization, the auto deduction will either deduce a std::initializer_list (if the types of entries in the braced-init-list are all identical) or be ill-formed otherwise.
Note that, auto a = {1, 2, 3}, b = {1}; remains unchanged and deduces initializer_list<int>. This change is intended as a defect resolution against C++14.
Now, let’s see the examples below.
Auto deduction from braced-init-list Rule #1
For direct list-initialization: For a braced-init-list with only a single element, the auto deduction will deduce from that entry. In the example below, there is a single member in the braced-init-list, and this is automatically defined as an initializer_list that consists of int members.
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auto a = { 30 }; // decltype(a) is std::initializer_list<int> for (auto i : a) std::cout << i << ","; std::cout << std::endl << std::endl; |
Auto deduction from braced-init-list Rule #2
For direct list-initialization: For copy-list-initialization, auto deduction will either deduce a std::initializer_list (if the types of entries in the braced-init-list are all identical) or be ill-formed otherwise. For a braced-init list with more than one element, the auto deduction will be ill-formed. Here is an example of creating an initilizer_list with an auto keyword,
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auto b = { 10 , 20 }; // decltype(b) is std::initializer_list<int> for (auto i : b) std::cout << i << ","; std::cout << std::endl << std::endl; |
Here above, a is automatically defined as an initializer_list that consists with int members, std::initializer_list<int>.
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auto bb = { 1.5f , 2.5f }; // decltype(bb) is std::initializer_list<float> for (auto i : bb) std::cout << i << ","; std::cout << std::endl << std::endl; |
Here above, aa is automatically defined as an initializer_list that consists with float members, std::initializer_list<float>.
Example below, gives error, because there are multiple element types (int and float), according to above “types of entries in the braced-init-list are all should be identical”
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auto bbb = { 1 , 2.0 }; // error: not a single element |
Auto deduction from braced-init-list Rule #3
For copy-list-initialization, auto deduction will either deduce a std::initializer_list. If the initialization is direct-list-initialization then the braced-init-list shall contain only a single initializer-clause L.
Before the C++17, auto a{1, 2, 3}, b{1}; was allowed and both variables were of type initializer_list<int>. Now auto a{1, 2, 3}; is ill-formed and auto b{1}; declares an int.
In this example below, c is declared as an int type,
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auto c{ 40 }; // decltype(c) is int std::cout << c << std::endl; |
in this concept, we can only list a single member, here example below gives error,
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// auto c{ 1, 2 }; // error: not a single element |
Is there a full example about auto deduction from braced-init-list in C++ 17?
Here is a full example about auto deduction from braced-init-list in C++17,
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#include <iostream> int main() { //Rule #1 auto a = { 30 }; // decltype(b) is std::initializer_list<int> for (auto i : a) std::cout << i << ","; std::cout << std::endl << std::endl; //Rule #2 auto b = { 10 , 20 }; // decltype(a) is std::initializer_list<int> for (auto i : b) std::cout << i << ","; std::cout << std::endl << std::endl; auto bb = { 1.5f , 2.5f }; // decltype(aa) is std::initializer_list<float> for (auto i : bb) std::cout << i << ","; std::cout << std::endl << std::endl; // auto bbb = { 1, 2.0 }; // error: cannot deduce element type //Rule #3 auto c{ 40 }; // decltype(c) is int std::cout << c << std::endl; // auto cc{ 1, 2 }; // error: not a single element system("pause"); return 0; } |
For more details, please see this https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3922.html
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