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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/api/basic_json/string_t/" rel="canonical"><link href=../start_pos/ rel=prev><link href=../swap/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>string_t - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/api/basic_json/string_t/" rel="canonical"><link href=../start_pos/ rel=prev><link href=../swap/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>string_t - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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</code></pre></div> <p>The type used to store <abbr title="JavaScript Object Notation">JSON</abbr> strings.</p> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes <abbr title="JavaScript Object Notation">JSON</abbr> strings as follows:</p> <blockquote> <p>A string is a sequence of zero or more Unicode characters.</p> </blockquote> <p>To store strings in C++, a type is defined by the template parameter described below. Unicode values are split by the <abbr title="JavaScript Object Notation">JSON</abbr> class into byte-sized characters during deserialization.</p> <h2 id=template-parameters>Template parameters<a class=headerlink href=#template-parameters title="Permanent link">¶</a></h2> <dl> <dt><code>StringType</code></dt> <dd> <p>the container to store strings (e.g., <code>std::string</code>). Note this container is used for keys/names in objects, see <a href=../object_t/ >object_t</a>.</p> <p><code>StringType</code> must have a <code>char</code>-compatible <code>value_type</code>: the library relies on <abbr title="Unicode Transformation Format">UTF</abbr>-8/<code>char</code>-based storage and processing internally, so <code>std::wstring</code>, <code>std::u16string</code>, and <code>std::u32string</code> are <strong>not</strong> valid choices for <code>StringType</code>. To work with wide-character data, convert it to/from <abbr title="Unicode Transformation Format">UTF</abbr>-8 at the boundary instead -- see the <abbr title="Frequently Asked Questions">FAQ</abbr>'s <a href=../../../home/faq/#wide-string-handling>wide string handling</a> section for a conversion recipe.</p> <p>Beyond the character type, the library expects a substantial part of the <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> interface (contiguous null-terminated <code>data()</code>, <code>substr()</code>, <code>find()</code>, <code>append()</code>, ...). See <a href=../../../features/types/template_parameters/#stringtype>Template Parameter Requirements</a> for the full list and for the string types that are known to work.</p> </dd> </dl> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <h3 id=default-type>Default type<a class=headerlink href=#default-type title="Permanent link">¶</a></h3> <p>With the default values for <code>StringType</code> (<code>std::string</code>), the default value for <code>string_t</code> is <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>.</p> <h3 id=encoding>Encoding<a class=headerlink href=#encoding title="Permanent link">¶</a></h3> <p>Strings are stored in <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoding. Therefore, functions like <code>std::string::size()</code> or <code>std::string::length()</code> return the number of bytes in the string rather than the number of characters or glyphs.</p> <h3 id=string-comparison>String comparison<a class=headerlink href=#string-comparison title="Permanent link">¶</a></h3> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> states:</p> <blockquote> <p>Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit, are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that <code>"a\\b"</code> and <code>"a\u005Cb"</code> are not equal.</p> </blockquote> <p>This implementation is interoperable as it does compare strings code unit by code unit.</p> <h3 id=storage>Storage<a class=headerlink href=#storage title="Permanent link">¶</a></h3> <p>String values are stored as pointers in a <code>basic_json</code> type. That is, for any access to string values, a pointer of type <code>string_t*</code> must be dereferenced.</p> <h3 id=cross-basic_json-conversion-requirements>Cross-<code>basic_json</code> conversion requirements<a class=headerlink href=#cross-basic_json-conversion-requirements title="Permanent link">¶</a></h3> <p>When converting a string value from one <code>basic_json</code> specialization to another via the <a href=../basic_json/ >converting constructor</a> (overload 4), the target <code>string_t</code> must be directly constructible from the source <code>basic_json</code>'s <code>string_t</code> type. If this requirement is not met, the conversion does not fail; instead, the string is silently converted as an array of character codes, which is incorrect. See <a href="https://github.com/nlohmann/json/issues/3425">issue #3425</a> for details and an example.</p> <h2 id=examples>Examples<a class=headerliLine truncated
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<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
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