dynamic type
时间: 2023-10-29 20:07:31 浏览: 38
Dynamic typing is a programming language feature that allows variables to be assigned without declaring their data type. In a dynamically typed language, the data type of a variable is determined at runtime rather than being explicitly declared at compile-time. This allows for more flexibility and ease of use, but can also lead to errors if the wrong data type is assigned to a variable. Some popular dynamically typed programming languages include Python, Ruby, and JavaScript.
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dynamic block
Dynamic blocks in Terraform allow you to create multiple instances of a resource with different configurations using a single block of code. This is useful when you need to create multiple similar resources, such as multiple EC2 instances with different instance types or different subnet IDs.
To create a dynamic block, you need to define a variable that contains a list of configurations, and then use a for_each loop to iterate over the list and create the resources. Here's an example of how to create multiple EC2 instances with different instance types:
```
variable "instance_types" {
type = list(string)
default = ["t2.micro", "t2.small", "t2.medium"]
}
resource "aws_instance" "ec2_instance" {
for_each = toset(var.instance_types)
ami = "ami-0c55b159cbfafe1f0"
instance_type = each.value
subnet_id = "subnet-0123456789abcdef"
}
```
In this example, the `instance_types` variable contains a list of three different instance types. The `for_each` loop iterates over the list and creates three EC2 instances with the specified instance types.
Dynamic blocks can also be used with other resource types, such as subnets or security groups. They provide a powerful way to create multiple resources with different configurations in a concise and efficient way.
dynamic_cast
dynamic_cast is an operator in C++ that is used for type casting. It is used to safely convert a pointer or reference to a class to a pointer or reference to a different class.
The syntax for dynamic_cast is:
```
dynamic_cast<new_type>(expression)
```
Here, new_type is the type to which the expression is being cast, and expression is the pointer or reference to the object being cast.
The dynamic_cast operator performs a runtime check to ensure that the cast is valid. If the cast is not valid, dynamic_cast returns a null pointer (for pointers) or throws a std::bad_cast exception (for references).
Dynamic_cast can only be used with pointers and references of polymorphic classes. A polymorphic class is a class that has at least one virtual function. If a class does not have any virtual functions, dynamic_cast cannot be used with it.