c++ - string::substr 中无法解释的 out_of_range

Question

在调用 substr 时，我遇到了一个关于 std::out_of_range 的恼人错误。准确的错误是

terminate called after throwing an instance of 'std::out_of_range'
what(): basic_string::substr

我绝对确定 tmp_request 的长度大于 1。无论我将什么传递给 substr — 1、2 或 bodypos — 它总是会抛出该错误。我在 Unix 上使用 g++。

我唯一可以包括的有趣的事情是字符串有多个 "\r\n"，包括一个 "\r\n\r\n"。

在一个cpp文件中:

std::string tmp_request, outRequest;

tmp_request = SS_Twitter->readData();
outRequest = SS_Twitter->parse(tmp_request);

在另一个:

 std::string parse(const std::string &request)
 {
  std::map<std::string,std::string> keyval;
  std::string outRequest;
  if(request[0]=='P')
  {
   if(request.find("register")!=std::string::npos)
   { //we have a register request
    size_t bodypos = request.find("username");
    if(bodypos==std::string::npos) 
    {
     HttpError(400,"Malformed HTTP POST request. Could not find key username.",request); 
    }
    else
    {
     std::string body = request.substr(bodypos);
     StringExplode(body,"&", "=",keyval);
     outRequest = "doing stuff";
    }

   }

更新:

std::string request2("P\r\nregister\r\nusername=hello\r\n\r\n");

std::string body = request2.substr(4);

这会引发相同的错误。现在我知道这是完全有效和正确的代码，但它仍然会抛出错误。 //删除源链接

Answer 1

我稍微修改了您的示例以减少使用的缩进量。
有 5 个“测试用例”，没有一个会导致任何问题。您能否提供一个示例请求来重现您遇到的问题。

编辑:忘记提及:如果这个示例(带有注释掉的位)没有产生该错误，那么您最好的选择是您的 StringExplode 函数中存在错误。您可以发布其来源，以获得更有帮助的建议。

编辑2: 在您的 StringExplode 中，将 results[tmpKey] = tmpKey.substr(found+1); 更改为 results[tmpKey] = tmpResult[i].substr(found +1);。将 int found 更改为 size_t found，并删除所有 if (found > 0)，这将修复您神秘的 out_of_range。您 substr-ing 了一个错误的字符串。为了以防万一，这里是修复代码:

void StringExplode(std::string str, std::string objseparator, std::string keyseperator,
                   std::map <std::string, std::string> &results)
{
    size_t found;
    std::vector<std::string> tmpResult;
    found = str.find_first_of(objseparator);
    while(found != std::string::npos)
    {
        tmpResult.push_back(str.substr(0,found));
        str = str.substr(found+1);
        found = str.find_first_of(objseparator);
    }
    if(str.length() > 0)
    {
        tmpResult.push_back(str);
    }

    for(size_t i = 0; i < tmpResult.size(); i++)
    {
        found = tmpResult[i].find_first_of(keyseperator);
        while(found != std::string::npos)
        {
                std::string tmpKey = tmpResult[i].substr(0, found);
                results[tmpKey] = tmpResult[i].substr(found+1);
                found = tmpResult[i].find_first_of(keyseperator, found + results[tmpKey].size());
        }

    }
}

初始测试代码:

#include <iostream>
#include <map>
#include <string>

std::string parse(const std::string &request)
{
    std::map<std::string,std::string> keyval;
    std::string outRequest;

    if(request[0] != 'P')
        return outRequest;

    if(request.find("register") == std::string::npos)
        return outRequest;

    //we have a register request
    size_t bodypos = request.find("username");
    if(bodypos==std::string::npos)
    {
        // HttpError(400,"Malformed HTTP POST request. Could not find key username.",request);
        // you said HttpError returns, so here's a return
        return outRequest;
    }

    std::string body = request.substr(bodypos);
    // StringExplode(body,"&", "=",keyval);
    outRequest = "doing stuff";

    return outRequest;
}

int main()
{

    std::string request("P\r\nregister\r\nusername=hello\r\n\r\n");
    std::cout << "[" << parse(request) << "]\n";

    request = "Pregisternusername=hello\r\n\r\n";
    std::cout << "[" << parse(request) << "]\n";

    request = "Pregisternusername=hello";
    std::cout << "[" << parse(request) << "]\n";

    request = "registernusername=hello";
    std::cout << "[" << parse(request) << "]\n";

    request = "";
    std::cout << "[" << parse(request) << "]\n";

    return 0;
}

这输出，可以预见:

[doing stuff]
[doing stuff]
[doing stuff]
[]
[]