C++ 的 std::string::size() 使用上要小心的地方

std::string::size() 的回傳會是 size_t 這樣的無號正整數或零的型別。

我在刷 LeetCode 的題目時寫出了這樣的程式碼。

class Solution {
public:
    int strStr(string haystack, string needle) {
        for (int i = 0; i <= haystack.size() - needle.size(); i++) {
        	// ...
        }
        return -1;
    }
};

當 needle 比 haystack 還要長時，其運算結果原以為會是負數，但是因為無號正整數或零的型別，就會被轉成正整數，導致 for 迴圈中會透過 i 去使用到 haystack 或是 needle 以外的記憶體位址，導致程式崩潰。

${shlibs:Depends} 使用上的一些陷阱

其實 0~git202411270842.3c1cdd3 這樣的 Debian Version 會等於 0~git202411270842.3c1cdd3-0

$ dpkg --compare-versions 0~git202411270842.3c1cdd3-0 eq 0~git202411270842.3c1cdd3; echo $?
0

於是 0~git202411270842.3c1cdd3-0~ 這樣的 Debian Version 就會小於 0~git202411270842.3c1cdd3

$ dpkg --compare-versions 0~git202411270842.3c1cdd3-0~ lt 0~git202411270842.3c1cdd3; echo $?
0

例如套件 A 使用了 0~git202411270842.3c1cdd3-0~ 這樣的版號。

陷阱來自於在套件 B 的 debian/control 裡面相依性使用 ${shlibs:Depends} 時，只會找到套件 A 的 upstream version 而已，然後會自動代入 (>= 0~git202411270842.3c1cdd3) 這樣的版號相依。

於是在套件 B 的 Debian packaging 打包後，就會遇到明明套件 B 的 Debian source packages 可以成功編譯成 Debian binary packages，但是卻無法安裝使用的情況。

$ sudo apt install libcamhal-ipu6ep
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libcamhal-ipu6ep : Depends: libbroxton-ia-pal-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libgcss-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-aiq-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-aiqb-parser-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-bcomp-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-cca-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-cmc-parser-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-coordinate-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-dvs-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-emd-decoder-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-exc-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-isp-bxt-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-lard-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-log-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-ltm-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-mkn-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
                    Depends: libia-nvm-ipu6ep0 (>= 0~git202411270842.3c1cdd3) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

至於解決方法就是避免使用比 0~git202411270842.3c1cdd3-0 還要小的版號，像是可以改用 0~git202411270842.3c1cdd3-1~ 這樣的版號就不會產生問題了。

$ dpkg --compare-versions 0~git202411270842.3c1cdd3-1~ gt 0~git202411270842.3c1cdd3; echo $?
0

以上是測試打包 ppa:oem-solutions-group/intel-ipu6 時的一些心得感想。