How to Fix Incorrect Function Parameter Passing in C++

Introduction When using Visual Studio 2019 version 16.11.1 for C++ development, you may encounter a perplexing issue where function parameters are incorrectly passed during optimization, leading to unexpected behavior. This is particularly noticeable when the optimization settings are adjusted for release builds. In this article, we will discuss the core issue, provide a minimal reproducible example, and explore potential solutions. Understanding the Issue The problem arises from the way the compiler optimizes function parameters when certain optimization flags are enabled. In simpler terms, the compiler sometimes makes assumptions about variable uses that can lead to undesired results. This can manifest in scenarios like the one described, where a member variable (x) of a class (foo) appears to be misrepresented after operations involving local variables and function calls. Example Code To illustrate this issue, let’s examine the example code provided: #include using namespace std; class foo { public: foo() { x = 32.0f; min_x = 4.0f; } void func(float val) { if (val < get_min_value()) { val = get_min_value(); } inner_func(val); } protected: void inner_func(float val) { float old = x; x = val; inner_print((int)old); } virtual float get_min_value() { return min_x; } void inner_print(int val) { cout

May 4, 2025 - 19:26

How to Fix Incorrect Function Parameter Passing in C++

Introduction

When using Visual Studio 2019 version 16.11.1 for C++ development, you may encounter a perplexing issue where function parameters are incorrectly passed during optimization, leading to unexpected behavior. This is particularly noticeable when the optimization settings are adjusted for release builds. In this article, we will discuss the core issue, provide a minimal reproducible example, and explore potential solutions.

Understanding the Issue

The problem arises from the way the compiler optimizes function parameters when certain optimization flags are enabled. In simpler terms, the compiler sometimes makes assumptions about variable uses that can lead to undesired results. This can manifest in scenarios like the one described, where a member variable (x) of a class (foo) appears to be misrepresented after operations involving local variables and function calls.

Example Code

To illustrate this issue, let’s examine the example code provided:

#include 

using namespace std;

class foo {
public:
    foo() {
        x = 32.0f;
        min_x = 4.0f;
    }

    void func(float val) {
        if (val < get_min_value()) {
            val = get_min_value();
        }

        inner_func(val);
    }

protected:
    void inner_func(float val) {
        float old = x;
        x = val;
        inner_print((int)old);
    }

    virtual float get_min_value() {
        return min_x;
    }

    void inner_print(int val) {
        cout << val << endl;
    }

private:
    float x;
    float min_x;
};

int main() {
    foo f;
    f.func(10.0f);

    return 0;
}

Break Down of the Functionality

Initialization
- The class foo initializes its member variable x to 32.0f.
Function Call
- The func method checks if the passed value is less than a minimum value and potentially adjusts it before passing it to the inner_func method.
Local Caching
- Inside inner_func, the original value of x is cached in the local variable old.
Printing the Value
- The expected output is 32, but under release build optimizations, it unexpectedly prints 10.

Why This Happens

When optimizations are enabled in Visual Studio, the compiler tries to reduce memory access and utilize registers more efficiently. In this scenario, it appears that the optimizer incorrectly handles the caching of x into old, leading to an incorrect print output.

Debug vs. Release Builds

It's important to note that this issue does not occur in debug builds. Debug builds typically disable various optimizations, allowing for a more straightforward behavior of the code. This discrepancy underscores the importance of testing code under both debug and release configurations, especially when performance optimizations are implemented.

Frequently Asked Questions

Why does this problem occur only in release builds?

Release builds enable various compiler optimizations that can alter the behavior of the code, particularly when local variables and class members interact in unexpected ways.

Is there a permanent fix to this issue?

As of now, workarounds are advisable, such as adjusting optimization settings or revising code structure. Monitoring for updates from Microsoft regarding this bug is recommended.

How can I report a suspected bug effectively?

When reporting a bug to Microsoft, provide a minimal reproducible example along with details of your environment, including the version of Visual Studio and any specific settings you've used.

Conclusion

Debugging strange behaviors in optimizations is a recurring challenge in C++ development. The example illustrated reveals potential drawbacks of compiler optimizations in Visual Studio 2019. Adjustments to optimizations, code structure, and keeping your development tools up-to-date can help navigate and resolve these issues effectively.