c ++模板问题[关闭]

Question

我目前正在使用本教程中给出的类：http://www.dreamincode.net/forums/topic/183191-create-a-simple-configuration-file-parser/

最初它运行良好，但由于我将单个源文件拆分为单独的头文件和 cpp 文件，我无法调用 getValueOfKey 函数

标题：

#ifndef CONFIGFILE_H
#define CONFIGFILE_H

#include <iostream>
#include <string>
#include <sstream>
#include <map>
#include <fstream>
#include <typeinfo>

class ConfigFile
{
private:
        std::map<std::string, std::string> contents;
    std::string fName;
    void removeComment(std::string &line) const;
    bool onlyWhitespace(const std::string &line) const;
    bool validLine(const std::string &line) const;
    void extractKey(std::string &key, size_t const &sepPos, const std::string &line) const;
    void extractValue(std::string &value, size_t const &sepPos, const std::string &line) const;
    void extractContents(const std::string &line);
    void parseLine(const std::string &line, size_t const lineNo);
    void ExtractKeys(); 
public:
    ConfigFile(const std::string &fName);
    bool keyExists(const std::string &key) const;
    template <typename ValueType>
    ValueType getValueOfKey(const std::string &key, ValueType const &defaultValue) const;
};


#endif  /* CONFIGFILE_H */

cpp:

#include "ConfigFile.h"

std::map<std::string, std::string> contents;
std::string fName;

template <typename T>
static std::string T_to_string(T const &val)
{
    std::ostringstream ostr;
    ostr << val;

    return ostr.str();
}

template <typename T>
static T string_to_T(std::string const &val)
{
    std::istringstream istr(val);
    T returnVal;
    if (!(istr >> returnVal))
        std::cout << "CFG: Not a valid " << (std::string)typeid (T).name() << " received!\n" << std::endl;

    return returnVal;
}

template <>
std::string string_to_T(std::string const &val)
{
    return val;
}

void ConfigFile::removeComment(std::string &line) const
{
    if (line.find(';') != line.npos)
        line.erase(line.find(';'));
}

bool ConfigFile::onlyWhitespace(const std::string &line) const
{
    return (line.find_first_not_of(' ') == line.npos);
}

bool ConfigFile::validLine(const std::string &line) const
{
    std::string temp = line;
    temp.erase(0, temp.find_first_not_of("\t "));
    if (temp[0] == '=')
        return false;

    for (size_t i = temp.find('=') + 1; i < temp.length(); i++)
        if (temp[i] != ' ')
            return true;

    return false;
}

void ConfigFile::extractKey(std::string &key, size_t const &sepPos, const std::string &line) const
{
    key = line.substr(0, sepPos);
    if (key.find('\t') != line.npos || key.find(' ') != line.npos)
        key.erase(key.find_first_of("\t "));
}

void ConfigFile::extractValue(std::string &value, size_t const &sepPos, const std::string &line) const
{
    value = line.substr(sepPos + 1);
    value.erase(0, value.find_first_not_of("\t "));
    value.erase(value.find_last_not_of("\t ") + 1);
}

void ConfigFile::extractContents(const std::string &line)
{
    std::string temp = line;
    temp.erase(0, temp.find_first_not_of("\t "));
    size_t sepPos = temp.find('=');

    std::string key, value;
    extractKey(key, sepPos, temp);
    extractValue(value, sepPos, temp);

    if (!keyExists(key))
        contents.insert(std::pair<std::string, std::string > (key, value));
    else
        std::cout << "CFG: Can only have unique key names!\n" << std::endl;
}

void ConfigFile::parseLine(const std::string &line, size_t const lineNo)
{
    if (line.find('=') == line.npos)
        std::cout << "CFG: Couldn't find separator on line: " << T_to_string(lineNo) << "\n" << std::endl;

    if (!validLine(line))
        std::cout << "CFG: Bad format for line: " << T_to_string(lineNo) << "\n" << std::endl;

    extractContents(line);
}

void ConfigFile::ExtractKeys()
{
    std::ifstream file;
    file.open(fName.c_str());
    if (!file)
        std::cout << "CFG: File " << fName << " couldn't be found!\n" << std::endl;

    std::string line;
    size_t lineNo = 0;
    while (std::getline(file, line))
    {
        lineNo++;
        std::string temp = line;

        if (temp.empty())
            continue;

        removeComment(temp);
        if (onlyWhitespace(temp))
            continue;

        parseLine(temp, lineNo);
    }

    file.close();
}

ConfigFile::ConfigFile(const std::string &fName)
{
    this->fName = fName;
    ExtractKeys();
}

bool ConfigFile::keyExists(const std::string &key) const
{
    return contents.find(key) != contents.end();
}

template <typename ValueType>
ValueType ConfigFile::getValueOfKey(const std::string &key, ValueType const &defaultValue = ValueType()) const
{
    if (!keyExists(key))
        return defaultValue;

    return string_to_T<ValueType> (contents.find(key)->second);
}

我尝试使用与单个文件相同的方法调用它，例如 std::cout << Config.getValueOfKey<std::string>("test");，但现在我收到以下编译器错误

main.cpp: In function 'int main(int, char**)':
main.cpp:29:71: error: no matching function for call to 'ConfigFile::getValueOfKey(const char [5])'
main.cpp:29:71: note: candidate is:
In file included from main.h:17:0,
                 from main.cpp:9:
ConfigFile.h:35:12: note: template<class ValueType> ValueType ConfigFile::getValueOfKey(const string&, const ValueType&) const
ConfigFile.h:35:12: note:   template argument deduction/substitution failed:
main.cpp:29:71: note:   candidate expects 2 arguments, 1 provided

鉴于我对模板的掌握不佳，我无法真正看到这个错误试图告诉我什么，我尝试传递直接字符串而不是 char 数组，但无济于事。任何帮助或解释将不胜感激，在过去的几个小时里，我的头在桌子上钻了一个漂亮的洞。

Answer 1

您声明了采用 2 个参数的方法：

ValueType getValueOfKey(const std::string &key, ValueType const &defaultValue) const;
//                                          |                           |
//                                   first parameter             second parameter

并且只提供一个：

Config.getValueOfKey<std::string>("test");

我还没有遇到一个编译器可以在没有任何帮助的情况下猜出你的意思。

您需要将默认值移到头文件中，并在其中声明方法：

ValueType getValueOfKey(const std::string &key, ValueType const &defaultValue = ValueType()) const;
    //                                          |                           |
    //                                   first parameter             second parameter

之后您可能会收到链接器错误，因此您可能需要检查this。