{"id":10203,"date":"2024-10-28T10:19:47","date_gmt":"2024-10-28T10:19:47","guid":{"rendered":"https:\/\/www.modernescpp.com\/?p=10203"},"modified":"2025-07-04T15:09:45","modified_gmt":"2025-07-04T15:09:45","slug":"placeholders-and-extended-character-set","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/placeholders-and-extended-character-set\/","title":{"rendered":"Placeholders and Extended Character Set"},"content":{"rendered":"\n

Placeholders are a nice way to highlight variables that are no longer needed. Additionally, the character set of C++26 will be extended.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Placeholders<\/h2>\n\n\n\n

Structured bindings are a C++17 feature that allows you to bind multiple variables to the elements of a structured object. <\/p>\n\n\n\n

The following program demonstrates using tuples and structured bindings to return and unpack multiple values from a function. <\/p>\n\n\n\n

\/\/ placeholder1.cpp<\/span>\n\n#include <tuple><\/span>\n#include <string><\/span>\n#include <iostream><\/span>\n\n\/\/ Function that returns three values<\/span>\nstd::<\/span>tuple<<\/span>int<\/span>, std::<\/span>string, double<\/span>><\/span> getThreeValues() {\n    int<\/span> intValue =<\/span> 42<\/span>;\n    std::<\/span>string strValue =<\/span> "example"<\/span>;\n    double<\/span> doubleValue =<\/span> 3.14<\/span>;\n    return<\/span> std::<\/span>make_tuple(intValue, strValue, doubleValue);\n}\n\nint<\/span> main() {\n\n    \/\/ Retrieve the three values using structured binding<\/span>\n    auto<\/span> [intValue, strValue, doubleValue] =<\/span> getThreeValues();\n\n    \/\/ Print the values<\/span>\n    std::<\/span>cout <<<\/span> "Integer: "<\/span> <<<\/span> intValue <<<\/span> '\\n'<\/span>;\n    std::<\/span>cout <<<\/span> "String: "<\/span> <<<\/span> strValue <<<\/span> '\\n'<\/span>;\n    std::<\/span>cout <<<\/span> "Double: "<\/span> <<<\/span> doubleValue <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre><\/div>\n\n\n\n
The function getThreeValues<\/code> is defined to return a tuple containing three different types of values: an int<\/code>, a std::string<\/code>, and a double<\/code>. These values are then packed into a tuple using std::make_tuple<\/code> and returned from the function.<\/p>\n\n\n\n
In the main<\/code> function, the program retrieves the three values returned by getThreeValues<\/code> using structured bindings. Structured bindings allow the program to unpack the tuple directly into three separate variables: intValue<\/code>, strValue<\/code>, and doubleValue<\/code>. This makes the code more readable and easier to work with than manually unpacking the tuple.<\/p>\n\n\n\n
 Sometimes, you don’t need all three values from the function getThreeValues<\/code>.<\/p>\n\n\n\n
\/\/ placeholder2.cpp<\/span>\n\n#include <tuple><\/span>\n#include <string><\/span>\n#include <iostream><\/span>\n\n\/\/ Function that returns three values<\/span>\nstd::<\/span>tuple<<\/span>int<\/span>, std::<\/span>string, double<\/span>><\/span> getThreeValues() {\n    int<\/span> intValue =<\/span> 42<\/span>;\n    std::<\/span>string strValue =<\/span> "example"<\/span>;\n    double<\/span> doubleValue =<\/span> 3.14<\/span>;\n    return<\/span> std::<\/span>make_tuple(intValue, strValue, doubleValue);\n}\n\nint<\/span> main() {\n\n    \/\/ Retrieve the three values using structured binding<\/span>\n    auto<\/span> [_, strValue, doubleValue] =<\/span> getThreeValues();\n\n    \/\/ Print the values<\/span>\n    std::<\/span>cout <<<\/span> "String: "<\/span> <<<\/span> strValue <<<\/span> '\\n'<\/span>;\n    std::<\/span>cout <<<\/span> "Double: "<\/span> <<<\/span> doubleValue <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre><\/div>\n\n\n\n
This time, the intValue <\/code>from the function getThreeValues<\/code> is not needed in the subsequent code. By convention, I bind it to the underline. <\/p>\n\n\n\n
At the same time, this means that the compiler does not issue a warning because the variable _ <\/code>is not used:<\/p>\n\n\n\n
Unfortunately, the intuitive _ can only be used once as an identifier. This changes with C++26. Now, it can be used as often as required.<\/p>\n\n\n\n
\/\/ placeholder3.cpp<\/span>\n\n#include <tuple><\/span>\n#include <string><\/span>\n#include <iostream><\/span>\n\n\/\/ Function that returns three values<\/span>\nstd::<\/span>tuple<<\/span>int<\/span>, std::<\/span>string, double<\/span>><\/span> getThreeValues() {\n    int<\/span> intValue =<\/span> 42<\/span>;\n    std::<\/span>string strValue =<\/span> "example"<\/span>;\n    double<\/span> doubleValue =<\/span> 3.14<\/span>;\n    return<\/span> std::<\/span>make_tuple(intValue, strValue, doubleValue);\n}\n\nint<\/span> main() {\n\n    \/\/ Retrieve the three values using structured binding<\/span>\n    auto<\/span> [_, strValue, _] =<\/span> getThreeValues();\n\n    \/\/ Print the values<\/span>\n    std::<\/span>cout <<<\/span> "String: "<\/span> <<<\/span> strValue <<<\/span> '\\n'<\/span>;\n    \n}\n<\/pre><\/div>\n\n\n\n
In this variant, neither the intValue <\/code>nor the doubleValue <\/code>from the function getThreeValues<\/code> is not required. I consistently use two underscores.<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Extended Character Set<\/h2>\n\n\n\n
Three new characters are available in the basic character set<\/a>:
<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
 The following program uses all three of them for raw string literals.<\/p>\n\n\n\n
\/\/ extendedCharacterset.cpp<\/span>\n\n#include <iostream><\/span>\n\nint<\/span> main<\/span>() {\n\n    std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n    auto<\/span> raw1 =<\/span> R"@(Hello<\/span>\\n<\/span>)@"<\/span>;    \n    auto<\/span> raw2 =<\/span> R"$(Hello<\/span>\\t<\/span>)$"<\/span>;    \n    auto<\/span> raw3 =<\/span> R"`(Hello<\/span>\\b<\/span>)`"<\/span>;\n    \n    std::<\/span>cout <<<\/span> "raw1: "<\/span> <<<\/span> raw1 <<<\/span> '\\n'<\/span>;    \n    std::<\/span>cout <<<\/span> "raw2: "<\/span> <<<\/span> raw2 <<<\/span> '\\n'<\/span>;    \n    std::<\/span>cout <<<\/span> "raw3: "<\/span> <<<\/span> raw3 <<<\/span> '\\n'<\/span>;\n\n    std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre><\/div>\n\n\n\n
The program then defines three raw string literals: raw1<\/code>, raw2<\/code>, and raw3<\/code>. Raw string literals in C++ are enclosed in R\"delimiter(...)delimiter\"<\/code>, where delimiter<\/code> can be any sequence of characters. This allows the string to contain special characters like \\n<\/code>, \\t<\/code>, and \\b<\/code> without needing to escape them.<\/p>\n\n\n\n
    \n
  • raw1<\/code> is defined as R\"@(Hello\\n)@\"<\/code>, which contains the text Hello\\n<\/code> without interpreting \\n<\/code> as a newline character.<\/li>\n\n\n\n
  • raw2<\/code> is defined as R\"$(Hello\\t)$<\/code>“, which contains the text Hello\\t<\/code> without interpreting \\t<\/code> as a tab character.<\/li>\n\n\n\n
  • raw3<\/code> is defined as R\"`(Hello\\b)`\"<\/code>, which contains the text Hello\\b<\/code> without interpreting \\b<\/code> as a backspace character.<\/li>\n<\/ul>\n\n\n\n
    Finally, here’s the output of the program:<\/p>\n\n\n\n
    \"\"<\/figure>\n\n\n\n
    What’s next?<\/h2>\n\n\n\n
    The core language of C++26 still offers improvements, such as pack indexing. I will write about this in the next blog post.<\/p>\n\n\n\n
    <\/p>\n","protected":false},"excerpt":{"rendered":"
    Placeholders are a nice way to highlight variables that are no longer needed. Additionally, the character set of C++26 will be extended. Placeholders Structured bindings are a C++17 feature that allows you to bind multiple variables to the elements of a structured object. The following program demonstrates using tuples and structured bindings to return and […]<\/p>\n","protected":false},"author":21,"featured_media":10211,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[559],"tags":[562],"class_list":["post-10203","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-c26-blog","tag-placeholders"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10203","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=10203"}],"version-history":[{"count":13,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10203\/revisions"}],"predecessor-version":[{"id":10261,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10203\/revisions\/10261"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/10211"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=10203"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=10203"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=10203"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}