C++ 循环:“;”标记之前的预期主表达式

C++ Loop: expected primary-expression before ‘;’ token

本文关键字：表达式 循环 C++      更新时间：2023-10-16

我有一个如下所示的循环，我认为问题出在标记为"请参阅此处"的内部 for 循环上。这有什么问题？

// measure time to write different sizes of data
for (int i = 0; i < sizeof(sizes)/sizeof(int); i++) {
    lengthMod = sizes[i]/sizeof(int) - 1;
    start = wall_clock_time();
    for (unsigned int j = 0; j < 10; j++) // << See here!!!
        tmp = 1;
    // force any write back cache to flush. read from other data source
    for (unsigned int j = 0; j < REPS; j++) // << Or here!!!
        tmp = j;
    end = wall_clock_time();
    timeTaken = ((float)(end - start))/1000000000;
    fprintf(stderr, "%d, %1.2f n", sizes[i]/1024, ((float)(end - start))/1000000000);
}

你可以在 GitHub 上看到完整的源代码 ~第 32 行

g++ -O3 cache-write.cpp -o cache-write -lrt
cache-write.cpp: In function ‘int main()’:
cache-write.cpp:36:27: error: expected primary-expression before ‘;’ token
cache-write.cpp:36:27: error: expected ‘)’ before ‘;’ token
cache-write.cpp:36:29: error: name lookup of ‘j’ changed for ISO ‘for’ scoping [-fpermissive]
cache-write.cpp:36:29: note: (if you use ‘-fpermissive’ G++ will accept your code)
cache-write.cpp:36:32: error: expected ‘;’ before ‘)’ token
cache-write.cpp:44:11: error: ‘data1’ was not declared in this scope
make: *** [cache-write] Error 1

不要在声明 REPS 时使用 #define，而是使用

const unsigned int REPS = whatever;

一般来说，C++避免使用 #define，因为它很容易导致你遇到的那种问题。

在 REPS 定义的末尾有一个分号：

#define REPS 128 * MB;
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