# C++ 函數中的默認參數 > 原文： [https://beginnersbook.com/2017/08/cpp-default-arguments/](https://beginnersbook.com/2017/08/cpp-default-arguments/) 在調用函數時不提供任何參數或僅提供少量參數時，將使用默認參數。在編譯程序期間使用默認參數。例如，假設您有一個[用戶定義的函數](https://beginnersbook.com/2017/08/cpp-functions/)`sum`聲明如下：`int sum(int a=10, int b=20)`，現在在調用此函數時，您不提供任何參數，簡稱為`sum()`;那么在這種情況下結果將是 30，編譯器使用函數簽名中聲明的默認值 10 和 20。如果你只傳遞一個這樣的參數：`sum(80)`那么結果將是 100，使用傳遞的參數 80 作為第一個值，20 個從默認參數中獲取。 ## 示例：C++ 中的默認參數 ```cpp #include <iostream> using namespace std; int sum(int a, int b=10, int c=20); int main(){ /* In this case a value is passed as * 1 and b and c values are taken from * default arguments. */ cout<<sum(1)<<endl; /* In this case a value is passed as * 1 and b value as 2, value of c values is * taken from default arguments. */ cout<<sum(1, 2)<<endl; /* In this case all the three values are * passed during function call, hence no * default arguments have been used. */ cout<<sum(1, 2, 3)<<endl; return 0; } int sum(int a, int b, int c){ int z; z = a+b+c; return z; } ``` **輸出：** ```cpp 31 23 6 ``` ## 默認參數的規則 正如您在上面的示例中所看到的，我在函數聲明期間僅為兩個參數`b`和`c`分配了默認值。您可以為所有參數或僅選定的參數指定默認值，但在僅為某些參數指定默認值時，請記住以下規則： > **如果為參數指定默認值，則必須為后續參數分配默認值，否則將出現編譯錯誤。** **例如：**讓我們看一些有效和無效的案例。 **有效：**以下函數聲明有效： ```cpp int sum(int a=10, int b=20, int c=30); int sum(int a, int b=20, int c=30); int sum(int a, int b, int c=30); ``` **無效：**以下函數聲明無效： ```cpp /* Since a has default value assigned, all the * arguments after a (in this case b and c) must have * default values assigned */ int sum(int a=10, int b, int c=30); /* Since b has default value assigned, all the * arguments after b (in this case c) must have * default values assigned */ int sum(int a, int b=20, int c); /* Since a has default value assigned, all the * arguments after a (in this case b and c) must have * default values assigned, b has default value but * c doesn't have, thats why this is also invalid */ int sum(int a=10, int b=20, int c); ```