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When programming in C++, variables are like storage boxes for holding data. Declaring a variable is essentially creating one of these boxes and giving it a name, so the program knows where to store or find the information later. In the C++ language, declaring a variable tells the compiler what kind of data the variable will hold and reserves the appropriate amount of memory.

To declare a variable, you must specify the data type and the variable's name in a single line. You can also declare multiple variables of the same type by separating their names with commas, like in this statement: ```cpp double temp, weight, height; ``` This single line efficiently declares three variables: `temp`, `weight`, and `height`, all of which are set to hold decimal values. By understanding how to declare variables, you can better organize and control data in your C++ programs.

Data types in C++ are used to define what kind of data can be stored in a variable. They guide the compiler on how much space to allocate in memory and decide how the stored data can be processed. There are several fundamental data types in C++:

• int - Represents whole numbers.
• double - For double-precision floating-point numbers. Typically used for decimal values, as seen with `temp`, `weight`, and `height` variables.
• char - For single characters, such as 'A', '&', or '3'.
• bool - For true/false values.
• float - For single-precision floating-point numbers. Less precise compared to double.

Using the correct data type is critical, as it affects how computations are performed. In the context of our variables `temp`, `weight`, and `height`, opting for `double` allows us to work accurately with decimal values, which is ideal for real-world measurements.

Programming syntax refers to the set of rules that defines the combinations of symbols that are considered to be correctly structured programs in a language. Syntax involves the arrangement of words and symbols to create proper expressions and statements.

In C++, syntax dictates how variables are declared, how loops are constructed, and how different structures within your code are pieced together to form a functioning program. Let's look at some basic syntax rules related to our original exercise:

• Semicolons are used to terminate statements. They act like periods in written language, marking the end of an instruction. For example: `double temp, weight, height;`
• Curly braces { } are used to group statements, often in the context of loops or functions.
• Names for variables should be descriptive and start with a letter or underscore (_), but not with a number. Avoid reserving keywords like `double`, `int`, etc.

Grasping syntax is fundamental as it ensures that your C++ programs are clear, efficient, and fail-safe from coding errors. By adhering to these syntax standards, coding becomes easier and more logical.