Initialize a C++ (11) std::vector with the content of another one and extra elements

Question

I need an "elegant" way of initializing a vector in the declaration phase with the content of another one and a few extra elements.

What I want to solve is:

Let's consider the following (example) declaration with initialization:

const std::vector<std::string> c90_types = {
    "char",
    "signed char",
    "unsigned char",
    "short",
    "unsigned short",
    "int",
    "unsigned int",
    "long",
    "unsigned long",
    "float",
    "double",
    "long double"
};

const std::vector<std::string> c99_types = {
    "char",
    "signed char",
    "unsigned char",
    "short",
    "unsigned short",
    "int",
    "unsigned int",
    "long",
    "unsigned long",
    "float",
    "double",
    "long double",
    "long long",
    "unsigned long long",
    "intmax_t",
    "uintmax_t"
};

as you can see c99_types has a subset which is exactly c90_types. I want to avoid the situation where I need to change the subset and then change manually the "superset" too, just to avoid the extra step that might introduce bugs :)

As a side note, I don't want to write code like:

second.insert(second.begin(), first.begin(), first.end());
second.push_back(something);

Any good and clean solutions to this?

Answer 1

There is a trick called "I want to initialize a const variable with something elaborate." that became possible with C++11, shamelessly stolen from Javascript.

const std::vector<std::string> c90_types = {
    "char",
    // and so on, and so forth....
};

const std::vector<std::string> c99_types = ([&](){
    const auto additional_types = { // initializer_list<const char *>, but it does not matter.
        "long long",
        "unsigned long long",
        "intmax_t",
        "uintmax_t"
    };

    std::vector<std::string> vec{c90_types};

    vec.insert(vec.end(), additional_types.begin(), additional_types.end());

    return vec;
})();

Pack your initialization logic into an unnamed lambda, and call it right away, copy-initializing your const variable.

vec is moved, not copied.

Answer 2

You could define the biggest vector (here that would be c99_types) first, and then construct the others with iterators from the largest one.

Here is an example :

const vector<int> a{1,2,3,4};
const vector<int> b{begin(a), begin(a)+2}; // b is {1,2}

So you could write:

const std::vector<std::string> c99_types = {
    "char",
    "signed char",
    "unsigned char",
    "short",
    "unsigned short",
    "int",
    "unsigned int",
    "long",
    "unsigned long",
    "float",
    "double",
    "long double",
    "long long",
    "unsigned long long",
    "intmax_t",
    "uintmax_t"
};

const std::vector<std::string> c90_types{begin(c99_types), begin(c99_types)+12};