cpp_template_stretchy_buffers

v1.cpp

template<typename T> struct buf_Meta {
    size_t len;
    size_t cap;
    T buf[0];
};

template <typename T> inline buf_Meta<T>& buf__meta(T*b) {
    return *(buf_Meta<T>*)((size_t)(b) - __builtin_offsetof(buf_Meta<T>, buf));
}

template <typename T> inline void buf_push(T*&b, T v) {
    if (!b) {
        b = (T*)((u8*)malloc(sizeof(T) + sizeof(buf_Meta<T>)) + sizeof(buf_Meta<T>)) ;
        buf__meta(b).len = 0;
        buf__meta(b).cap = 1;
    }
    
    buf__meta(b).len++;
    if (buf__meta(b).len > buf__meta(b).cap) {
        buf__meta(b).cap = buf__meta(b).len;
        
        void* base = &buf__meta(b);
        base = realloc(base, buf__meta(b).cap * sizeof(T) + sizeof(buf_Meta<T>)) ;
        assert(base);
        b = (T*)((size_t)base + sizeof(buf_Meta<T>));
        
    }
    b[buf__meta(b).len-1] = v;
}

template <typename T> inline size_t buf_len(T*b) {
    return buf__meta(b).len;
}

v2.cpp

template<typename T> struct DynamicArray { 
    size_t len = 0;
    size_t cap = 0;
    T     *buf = 0;
    void  cap_set(size_t new_cap)  { 
        system_alloc(buf, new_cap);  
        cap = new_cap; 
    }
    auto& append() { 
        len++; 
        if (len > cap) cap_set(MAX(len, cap *2)); 
        return buf[len-1]; 
    }
    auto& append    (T v     ) { return append() = v;  }
    auto& operator[](size_t i) { assert(i<len); return buf[i];        }
    ~DynamicArray   (        ) { cap_set(0);           }
    auto begin      (        ) { return buf;           }
    auto end        (        ) { return buf + len;     }
};