C ++ std :: stringstream运算符<< vs（构造函数）

Question

My expectation is that I can create an uninitialized std::stringstream and feed it a string or just create it with the same stream initially and get the same results. 我的期望是，我可以创建一个未初始化的std::stringstream并为它提供一个字符串，或者只是最初使用相同的流创建它并获得相同的结果。 When I did the first case the code was working as I expected. 当我做第一种情况时，代码按预期工作。 Then I tried the second case expecting there would be the same result which did not happen. 然后，我尝试了第二种情况，期望会有相同的结果，但不会发生。 What am I missing? 我想念什么？

Fragment of the first case. 第一种情况的片段。

...
int main() {
constexpr auto APTCP_XML_DOC_PREFIX            {R"EOD(<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE doc [<!ENTITY nbsp "&#xa0;">]>
<doc>
<head>
</head>
<body>
)EOD"};
   std::stringstream xml_doc; xml_doc << APTCP_XML_DOC_PREFIX;
...
   if (transformer.transform(xml_doc, style_sheet, std::cout) != 0)
      std::cerr << "aptcp/main()/transformer.getLastError(): " << transformer.getLastError() << "\n" << style_sheet.str() << xml_doc.str();
   }

Second case has xml_doc initialized this way instead. 第二种情况是xml_doc这种方式初始化了xml_doc 。

   std::stringstream xml_doc(APTCP_XML_DOC_PREFIX);

with this error: 出现此错误：

Fatal Error: comment or processing instruction expected (Occurred in an unknown entity, at line 2, column 1.)
aptcp/main()/transformer.getLastError(): SAXParseException: comment or processing instruction expected (Occurred in an unknown entity, at line 2, column 1.)
<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet id="aptcp_stylesheet"
                version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output
            method="html"
            version="4.01"
            indent="yes"
            doctype-system="http://www.w3.org/TR/html4/strict.dtd"
            doctype-public="-//W3C//DTD HTML 4.01//EN"/>
<xsl:template match="xsl:stylesheet" />
<xsl:param name="current_time_p"/>
<xsl:template match="/">
  <html>
  <body>
    <style>
    pp {display: none;}
    pp, table, tr {
        width: 100%;
...

Answer 1

This code: 这段代码：

#include <iostream>
#include <sstream>
#include <string>
#include <utility>
#include <cstring>

int main() {
    constexpr auto BEFORE {"Before"};

    std::stringstream un; un << BEFORE;
    un << "UN";
    std::cout << "un=" << un.str() << "." << std::endl;

    std::stringstream con(BEFORE);
    con << "CON";
    std::cout << "con=" << con.str() << "." << std::endl;
    }

shows the value is placed at the front rather than at the end. 显示该值放在前面而不是结尾。

un=BeforeUN.
con=CONore.

To answer the question, the mode is missing ( std::ios::ate "seek to the end of stream immediately after open" ): 要回答这个问题，缺少模式（ std::ios::ate “打开后立即搜索到流的末尾” ）：

std::stringstream xml_doc(APTCP_XML_DOC_PREFIX,
    std::ios::in|std::ios::out|std::ios::ate);

C ++ std :: stringstream运算符<< vs（构造函数）

问题描述

1 个解决方案

解决方案1
1 已采纳 2018-04-09 22:47:40

C ++ std :: stringstream运算符&lt;&lt; vs（构造函数）

问题描述

1 个解决方案

解决方案1 1 已采纳 2018-04-09 22:47:40

C ++ std :: stringstream运算符<< vs（构造函数）

解决方案1
1 已采纳 2018-04-09 22:47:40