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  • Author Author: Jan Cumps
  • Date Created: Date Created
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Add external SPI Flash Memory to Raspberry Pico - 5: pass STL containers as read-only parameters

Jan Cumps
Add external SPI Flash Memory to Raspberry Pico - 5: pass STL containers as read-only parameters

A projects to learn the SPI API of the RP2040 C SDK, and to use it to control an external Flash IC. In this post, I refine the m25 c++ class again. take care that an STL container is read-only when passing it to a member.

image

Pass an Array as read-only input parameter

C++ can be used as a strict language. One of the strict options is to flag that an object, passed as reference to a method, will / can not be modified inside the method. The object will have the same state after the execution as before the execution.

class m25 {
public:
  // ...
  void write(uint32_t address, const std::vector<uint8_t>& data);

In the write() method, the first parameter is passed by value. That is non-mutable by design.
The second parameter is a reference to a vector object. I pass it by reference, to avoid that a copy has to be made of this object. This is a common c++ construct. But that gives the write() the powers to alter the vector or its content.

When you add const in front of the parameter, it's a sign for the compiler that you can only perform calls on that object that don't alter it. When you try to call a modifier (add or change date, empty the container, ...), the compiler will throw an error.

That's it - a small construct that can help avoid that an object is altered. There's more to const. See from page 66 on in Industrial Strength c++.

image

link to all posts in this series

  • in reply to Jan Cumps

    link to the quote above: https://cplusplus.com/reference/vector/vector/operator[]/

    (I couldn't edit the comment)

  • side note:
    In my write() method, I access the raw data buffer of the vector, via the [] operator:

    std::vector<uint8_t> m25::rdid() {
    // ....
    spi_read_blocking(spi, 0, (&v[0]), v.size()); // Manufacturer Identification, Device Identification (Memory Type || Memory Capacity)
    // ...
}

    You could think that this may give my method the powers to modify the raw buffer. But there is protection:

    If the vector object is const-qualified, the function returns a const_reference. Otherwise, it returns a reference.

    In my code, I pass the vector as a const reference. So it's const-qualified and the raw buffer can only be used in code that doesn't alter it.