{"id":4251,"date":"2021-03-23T13:49:21","date_gmt":"2021-03-23T11:49:21","guid":{"rendered":"https:\/\/mariusbancila.ro\/blog\/?p=4251"},"modified":"2021-05-19T09:51:04","modified_gmt":"2021-05-19T07:51:04","slug":"initializing-statement-for-if-switch-foreach","status":"publish","type":"post","link":"https:\/\/mariusbancila.ro\/blog\/2021\/03\/23\/initializing-statement-for-if-switch-foreach\/","title":{"rendered":"Initializing statement for if\/switch\/foreach"},"content":{"rendered":"\n

There are several statements in C++ whose syntax was modified in recent versions of the standard. I refer here to the if and switch statements that were modified in C++17 to include initializing statements, and the range-based for loop that supports the same as of C++20. Their general form is shown in the following table:<\/p>\n\n\n\n

C++17<\/td>if(init;opt<\/em><\/sub> condition)<\/code><\/td><\/tr>
C++17<\/td>switch(init;opt<\/em><\/sub> condition)<\/code><\/td><\/tr>
C++20<\/td>for(init;opt<\/em><\/sub> declaration : initializer)<\/code><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The init<\/code> part is optional and, if missing, then we have the regular (or traditional) form of the statement. <\/p>\n\n\n\n

I am going to show a few examples here, but before I go further, I must make a small comment. There is no such thing as foreach<\/em> in C++. At least, not as a keyword\/statement. There is the concept of a for each statement, but it’s called range-based for loop<\/em>. I always disliked this very long name, and I prefer to call it, at least in simplified context, a “foreach”.<\/p>\n\n\n\n

if statements<\/h2>\n\n\n\n

Let’s start with if statements. The following listing shows snippets before C++17.<\/p>\n\n\n\n

auto val = get_value();\nif(val > 0)\n{    \n   std::cout << \"processing \" << val << '\\n';\n}\nelse \n{\n   std::cout << \"value \" << val << \" is illegal\\n\";\n}\n\nstd::set<int> n{ 1,2,3,5,8 };\nauto pos = n.find(3);\nif (pos != std::end(n))\n{\n   \/* do something *\/\n}<\/pre>\n\n\n\n
And this is how these snippets can be simplified with init statements in C++17:<\/p>\n\n\n\n
if (auto val = get_value(); val > 0)\n{    \n   std::cout << \"processing \" << val << '\\n';\n}\nelse \n{\n   std::cout << \"value \" << val << \" is illegal\\n\";\n}\n\nstd::set<int> n{ 1,2,3,5,8 };\nif (auto pos = n.find(3); pos != std::end(n))\n{    \n   \/* do something *\/ \n}<\/pre>\n\n\n\n
A statement of the form if(init; condition) statement;<\/code> is equivalent to the following:<\/p>\n\n\n\n
{\n   init;\n   if(condition)\n      statement;\n}<\/pre>\n\n\n\n
Therefore, the variable, or variables (because you can define multiple variables of the same type, separated by comma) are declared within a scope that immediately encloses the if statement. This means they are available not only in the if statement but also in all ensuing else-if statements. These may have other init statements, but you cannot redeclare a variable declared in the same if. Here is another example:<\/p>\n\n\n\n
if (auto a = get_value(); a > 0)\n   std::cout << \"processing A \" << a << '\\n';\nelse if(auto b = get_another_value(); b > 0)\n   std::cout << \"processing B \" << b << '\\n';<\/pre>\n\n\n\n
This code is transformed by the compiler into the following form:<\/p>\n\n\n\n
{\n   auto a = get_value();\n   if (a > 0)\n   {\n      std::cout << \"processing A \" << a << '\\n';\n   }\n   else\n   {\n      auto b = get_another_value();\n      if(b > 0)\n      {\n         std::cout << \"processing B \" << b << '\\n';\n      }\n   }\n}<\/pre>\n\n\n\n
switch statements<\/h2>\n\n\n\n
The other statement that supports init statements in C++17 is the switch statement. It’s syntax is very similar to what we have seen before.<\/p>\n\n\n\n
Before C++17, a switch statement would typically have the following form (this being a simple example):<\/p>\n\n\n\n
auto option = get_option();\nswitch (option)\n{\ncase 'a': \/* add *\/   break;\ncase 'd': \/* del *\/   break;\ncase 'l': \/* list *\/  break;\ncase 'q': \/* quit *\/  break;\ndefault:  \/* error *\/ break;\n}<\/pre>\n\n\n\n
In C++17, this statement can be refactored to the following form:<\/p>\n\n\n\n
switch (auto option = get_option(); option)\n{\n   case 'a': \/* add *\/   break;\n   case 'd': \/* del *\/   break;\n   case 'l': \/* list *\/  break;\n   case 'q': \/* quit *\/  break;\n   default:  \/* error *\/ break;\n}<\/pre>\n\n\n\n
Like in the case of the if statement, the compiler introduces an enclosing scope, transforming the above snippet to the following form:<\/p>\n\n\n\n
{\n   auto option = get_option();\n   switch (option)\n   {\n      case 'a': \/* add *\/   break;\n      case 'd': \/* del *\/   break;\n      case 'l': \/* list *\/  break;\n      case 'q': \/* quit *\/  break;\n      default:  \/* error *\/ break;\n   }\n}<\/pre>\n\n\n\n
range-based for loop statements<\/h2>\n\n\n\n
The latest statement to support initialization statements is the range-based for loops. This change was introduced in C++20 and is very similar to what we have seen so far.<\/p>\n\n\n\n
The following is a range-based for loop prior to C++20:<\/p>\n\n\n\n
std::vector<int> get_values() { return { 1,2,3,5,8 }; }\n\nauto const& values = get_values();\nfor (auto const& v : values)\n   std::cout << v << '\\n';<\/pre>\n\n\n\n
In C++20, we can rewrite this to the following form:<\/p>\n\n\n\n
for (auto const& values = get_values(); auto const& v : values)\n   std::cout << v << '\\n';<\/pre>\n\n\n\n
Again, the compiler transforms it by adding an enclosing scope:<\/p>\n\n\n\n
{\n   auto const& values = get_values();\n   for (auto const& v : values)\n      std::cout << v << '\\n';\n}<\/pre>\n\n\n\n
Actually, this is just a simplified form, since the compiler is transforming the range-based for loop at the same time. The more correct form is shown below:<\/p>\n\n\n\n
{\n    auto const& values = get_values();  \/\/ with std::vector\n    auto &&__range = values;\n    auto __begin = values.begin();\n    auto __end = values.end();\n    for ( ; __begin != __end; ++__begin ) {\n        auto const& v = *__begin;\n        std::cout << v << '\\n';\n    }\n}<\/pre>\n\n\n\n
This form with init statement can be handy when you iterate over a range but you also need the index of the elements of the range. Here is an example:<\/p>\n\n\n\n
for (size_t index = 1; auto const& v : get_values())\n   std::cout << \"value \" << index++ << \": \" << v << '\\n';<\/pre>\n\n\n\n
Initialization statements for range-based for loops are available in GCC 9, Clang 8, and visual Studio 2019 16.5.<\/p>\n\n\n\n
If you want to see how the compiler is transforming your code you can try your snippets at https:\/\/cppinsights.io\/<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
There are several statements in C++ whose syntax was modified in recent versions of the standard. I refer here to the if and switch statements that were modified in C++17 to include initializing statements, and the range-based for loop that supports the same as of C++20. Their general form is shown in the following table:<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","advgb_blocks_editor_width":"","advgb_blocks_columns_visual_guide":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[7],"tags":[451,475,543],"class_list":["post-4251","post","type-post","status-publish","format-standard","hentry","category-c","tag-c","tag-c17","tag-c20"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t