std::vector initialization

main.cpp

#include <cstdio>
#include <memory>
#include <string>
#include <typeinfo>
#include <vector>

#include <cxxabi.h>

template<typename T>
std::string getType(const T & t)
{
    int status = 0;
    const auto & info = typeid(t);
    char * type = abi::__cxa_demangle(info.name(), 0, 0, &status);
    std::string result((status == 0) ? type : info.name());
    free(type);
    return result;
}

template<typename T>
void print(const char* varname, const T & var)
{
    printf("%s [%3zu] : %s\n", varname, var.size(), getType(var).c_str());
}

#define P(var) print(#var, var)

int main()
{
    // https://godbolt.org/z/Gq7oG39qE

    // 1. vector of 10 integers
    std::vector<int> a1(10);
    // 2. vector of 10 integers
    std::vector<int> a2(10, 1);
    // 3. vector of 1 integer
    std::vector<int> a3{10};
    // 4. vector of 2 integers
    std::vector<int> a4{10, 1};

    // error: narrowing conversion
//  std::vector<char> b1{128, 'a'};
    // 5. vector of 2 chars
    std::vector<char> b2{127, 'a'};
    // 6. vector of 128 strings
    std::vector<std::string> b3{128, "a"};

    // 7. vector of 10 integers (a copy of a1)
    std::vector<int> c1{a1.begin(), a1.end()};
    // 8. vector of 2 iterators (C++17 required from now on, for Class Template Argument Deduction)
    std::vector      c2{a1.begin(), a1.end()};

    // 9. vector of 10 integers (a copy of a1)
    std::vector d1(a1);
    // 10a. vector of 1 vector in gcc <= 7.5 and clang <=5
    // 10b. vector of 10 integers (a copy of a1) in gcc >= 8.1 and clang >= 6
    std::vector d2{a1};
    // 11. vector of 2 vectors
    std::vector d3{a1, a1};

    P(a1);
    P(a2);
    P(a3);
    P(a4);

//  P(b1);
    P(b2);
    P(b3);

    P(c1);
    P(c2);

    P(d1);
    P(d2);
    P(d3);
}