typedef struct _t {
    std::string         id;
    std::vector< char > body;
} t;

explicit vector ( size_type n, const T& value= T(), const Allocator& = Allocator() );

#include <iostream>
#include <vector>

class Foo
{
public:
    Foo() : myvec(1024) {}
    void foo() { std::cout << myvec.size() << ' ' << myvec.capacity() << '\n'; }

private:
    std::vector<int> myvec;
};

int main()
{
    Foo instance;
    instance.foo();
}

KP>explicit vector ( size_type n, const T& value= T(), const Allocator& = Allocator() );
KP>

typedef struct _t {
    std::string         id;
    std::vector< char > body;
} t;
 
vector<t> a;

typedef struct _t {
    std::string         id;
    std::vector< char > body;

   _t() : body(1024) { }

} t;