C++ 未知符号 - 难以发现 .h 和 .cpp 文件之间的差异

Question

在我的 C++ 程序中，出现了这个错误：

Undefined symbols for architecture x86_64:
"MathExpression::layerFunctions", referenced from:
MathExpression::initializeLayerFunctions() in MathExpression.cpp.o
MathExpression::layer(MathExpression&, MathExpression::Operation, 
std::__1::vector<short, std::__1::allocator<short> >&) in 
MathExpression.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see 
invocation)
make[3]: *** [MathTestGenerator] Error 1
make[2]: *** [CMakeFiles/MathTestGenerator.dir/all] Error 2
make[1]: *** [CMakeFiles/MathTestGenerator.dir/rule] Error 2
make: *** [MathTestGenerator] Error 2

我的头文件中有这个声明：

static std::vector<std::function<void(MathExpression &, std::vector<NumberType> &)>> layerFunctions;

在我的 .cpp 文件中，我有：

std::vector<std::function<void(MathExpression &, std::vector<MathExpression::NumberType> &)>> layerFunctions(static_cast<MathExpression::OperationType> (MathExpression::Operation::EOE));

连同此功能：

void MathExpression::initializeLayerFunctions() {
layerFunctions.resize(static_cast<OperationType>(Operation::EOE));

layerFunctions[static_cast<unsigned long>(Operation::addition)] = [] (MathExpression & exp, std::vector<NumberType> & otherArgs) -> void {
exp.string.insert(exp.string.end(), {' ', operationToChar(Operation::addition),    ' ', static_cast<CharType> 
(otherArgs[0])});
};

layerFunctions[static_cast<unsigned long>(Operation::subtraction)] = [] (MathExpression & exp, std::vector<NumberType> & otherArgs) -> void {
exp.string.insert(exp.string.end(), {' ', operationToChar(Operation::subtraction), ' ', static_cast<CharType> 
(otherArgs[0])});
};

layerFunctions[static_cast<unsigned long>(Operation::EOE)] = [] (MathExpression & exp, std::vector<NumberType> & otherArgs) -> void {
    // Throw or assert or something.
     };
}

我不确定自己做错了什么。据我所知，我的声明是匹配的，但我无法摆脱错误。有人会对如何解决这个问题有见识吗？未来感谢。

作为参考，这里分别是我的头文件和 .cpp 文件（请记住，这是我的第一个 C++ 程序，所以我确定缺少约定）：

#ifndef MATHTESTGENERATOR_MATHEXPRESSION_H
#define MATHTESTGENERATOR_MATHEXPRESSION_H

#include <vector>
#include <functional>

class MathExpression {

public:
    using FieldType     = unsigned char;
    using OperationType = unsigned char;
    using NumberType    = short int;
    using CharType      = char;

    enum class Field : FieldType {
        integers,
        EOE // rational, real, complex.
    };

    enum class Operation : OperationType {
        addition,
        subtraction,
        EOE // multiplication, division, absolute value, radical
    };

    explicit MathExpression(std::vector<CharType>);
    std::vector<CharType> string;

    static void print(MathExpression &);
    static void layer(MathExpression &, Operation, 
    std::vector<NumberType> &);

    static void initialize();

    private:
    static char operationToChar(OperationType);
    static char operationToChar(Operation);

    static std::vector<std::function<void(MathExpression &, 
std::vector<NumberType> &)>> layerFunctions;
    static void initializeLayerFunctions();
};

#endif //MATHTESTGENERATOR_MATHEXPRESSION_H

和

#include "MathExpression.h"
#include <list>
#include <iostream>

std::vector<std::function<void(MathExpression &, 
std::vector<MathExpression::NumberType> &)>> 
layerFunctions(static_cast<MathExpression::OperationType>, (MathExpression::Operation::EOE));


void MathExpression::initialize() {
initializeLayerFunctions();
}

void MathExpression::initializeLayerFunctions() {
layerFunctions.resize(static_cast<OperationType>(Operation::EOE));

layerFunctions[static_cast<unsigned long>(Operation::addition)]    = [] (MathExpression & exp, std::vector<NumberType> & otherArgs) -> void { 
exp.string.insert(exp.string.end(), {' ', operationToChar(Operation::addition),    ' ', static_cast<CharType> 
(otherArgs[0])});
};

layerFunctions[static_cast<unsigned long>(Operation::subtraction)] = [] 
(MathExpression & exp, std::vector<NumberType> & otherArgs) -> void {
exp.string.insert(exp.string.end(), {' ', operationToChar(Operation::subtraction), ' ', static_cast<CharType> 
(otherArgs[0])});
};

layerFunctions[static_cast<unsigned long>(Operation::EOE)]         = [] (MathExpression & exp, std::vector<NumberType> & otherArgs) -> void {
// Throw or assert or something.
};
}

char MathExpression::operationToChar(OperationType ordinal) {
return operationToChar(static_cast<Operation>(ordinal));
}
char MathExpression::operationToChar(Operation op) {
switch(op) {
    case Operation::addition    : return '+';
    case Operation::subtraction : return '-';
    default                     : return '_';
}
}

MathExpression::MathExpression(std::vector<CharType> exp) {
this->string = std::vector<CharType>(exp);
}

void MathExpression::print(MathExpression &exp) {
for(int i = 0; i < exp.string.size(); i++) {
    std::cout << exp.string[i];
}
}

void MathExpression::layer(MathExpression &exp, 
MathExpression::Operation op, std::vector<NumberType> &otherArgs) {
layerFunctions[static_cast<OperationType>(op)](exp, otherArgs);
}

Answer 1

问题出在 MathExpression.CPP 内。您定义了 layerFunctions，但由于它不在 MathExpressions 命名空间内，I.E. MathExpressions::layerFunctions，它说你的 header 里面有一个链接器错误。通过将该名称空间添加到您的函数声明中，您的问题应该不再是问题。如果您快速 google.

，您可以到处找到有关链接器错误的信息

C++ 未知符号 - 难以发现 .h 和 .cpp 文件之间的差异

C++ Unknown Symbol - Difficulty Finding Discrepancy Between .h and .cpp File

linker

header-files

c++11