如何在C ++中的构造函数中定义extern const

Question

Two config.json files hold values that I want to store into constants. My plan was to load and parse the json file in the constructor of the corresponding classes and assign the values to constants. However, in debugging mode, the error " ... LNK2019: unresolved external symbol..." occurs.

So, is it not possible to define constants in a constructor?

Since the values of the constants depend on the variable VARIANT , I've made a ifelse to read the correct .json file.

// constants.h
extern const int    BARL;
extern const int BAR_TOL;

// constants.cpp
Constants::Constants() {
    Json::Reader reader;
    Json::Value root;

    #if VARIANT == A
        std::ifstream config_a_file("a.json");
        reader.parse(config_a_file, root);
    #elif VARIANT == B
        std::ifstream config_b_file("b.json");
        reader.parse(config_b_file, root);
    #endif

    const int   BARL = root["BARL"].asInt();
    const int   BAR_TOL = BARL * 3;
}

What I expect is that the constants BARL and BAR_TOL can be used in the entire constants.cpp class. However, their definition cannot be found.

Answer 1

When declaring external variables, the scope they appear in must match the scope they are defined in, to the namespace level. And you can't define two globals with external linkage in block scope at all.

Putting aside the discussion ¹ about why you should be careful with such global constants, I'd say you are going about this wrong. Since you have a Constants class, just expose those constants as members, and declare a single external Constants instance. Something like:

struct Constants {
    int    BARL;
    int BAR_TOL;
    Constants();
};

extern Constants const constants;

Then other code may deal with the better namespaced constants.BARL , which is itself a constant since it's a sub-object of the const constants object.

The corresponding cpp file then becomes this:

Constants const constants; // definition

Constants::Constants() {
    Json::Reader reader;
    Json::Value root;

    #if VARIANT == A
        std::ifstream config_a_file("a.json");
        reader.parse(config_a_file, root);
    #elif VARIANT == B
        std::ifstream config_b_file("b.json");
        reader.parse(config_b_file, root);
    #endif

    BARL = root["BARL"].asInt(); // assignment
    BAR_TOL = BARL * 3;
}

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_{¹ - For that, you can browse static-order-fiasco for many examples about why the unspecified initialization order between static variables in different translation units can cause headaches.}